[PATCH] rust: fix building firmware abstraction on 32bit arm

[Christian Schrefl <chrisi.schrefl@gmail.com>]

When trying to build the rust firmware abstractions on 32 bit arm the
following build error occures:

```
error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:20:14
   |
20 |         Self(bindings::request_firmware)
   |         ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _`
                 found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {request_firmware}`
note: tuple struct defined here
  --> rust/kernel/firmware.rs:14:8
   |
14 | struct FwFunc(
   |        ^^^^^^

error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:24:14
   |
24 |         Self(bindings::firmware_request_nowarn)
   |         ---- ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found fn item
   |         |
   |         arguments to this function are incorrect
   |
   = note: expected fn pointer `unsafe extern "C" fn(_, *const i8, _) -> _`
                 found fn item `unsafe extern "C" fn(_, *const u8, _) -> _ {firmware_request_nowarn}`
note: tuple struct defined here
  --> rust/kernel/firmware.rs:14:8
   |
14 | struct FwFunc(
   |        ^^^^^^

error[E0308]: mismatched types
  --> rust/kernel/firmware.rs:64:45
   |
64 |         let ret = unsafe { func.0(pfw as _, name.as_char_ptr(), dev.as_raw()) };
   |                            ------           ^^^^^^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`
   |                            |
   |                            arguments to this function are incorrect
   |
   = note: expected raw pointer `*const i8`
              found raw pointer `*const u8`

error: aborting due to 3 previous errors
```

To fix this error the char pointer type in `FwFunc` is converted to
`ffi::c_char`.

Fixes: de6582833db0 ("rust: add firmware abstractions")
Cc: stable@vger.kernel.org # Backport only to 6.15 needed

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
---
 rust/kernel/firmware.rs | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/rust/kernel/firmware.rs b/rust/kernel/firmware.rs
index f04b058b09b2d2397e26344d0e055b3aa5061432..1d6284316f2a4652ef3f76272670e5e29b0ff924 100644
--- a/rust/kernel/firmware.rs
+++ b/rust/kernel/firmware.rs
@@ -5,14 +5,18 @@
 //! C header: [`include/linux/firmware.h`](srctree/include/linux/firmware.h)
 
 use crate::{bindings, device::Device, error::Error, error::Result, str::CStr};
-use core::ptr::NonNull;
+use core::{ffi, ptr::NonNull};
 
 /// # Invariants
 ///
 /// One of the following: `bindings::request_firmware`, `bindings::firmware_request_nowarn`,
 /// `bindings::firmware_request_platform`, `bindings::request_firmware_direct`.
 struct FwFunc(
-    unsafe extern "C" fn(*mut *const bindings::firmware, *const u8, *mut bindings::device) -> i32,
+    unsafe extern "C" fn(
+        *mut *const bindings::firmware,
+        *const ffi::c_char,
+        *mut bindings::device,
+    ) -> i32,
 );
 
 impl FwFunc {

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250408-rust_arm_fix_fw_abstaction-4c3a89d75e29

Best regards,
-- 
Christian Schrefl <chrisi.schrefl@gmail.com>