FTBFS: Rust firmware abstractions in current stable (6.13.4) on arm64 with rustc 1.85.0

[NoisyCoil <noisycoil@disroot.org>]

Hi!

The Rust firmware abstractions FTBFS on arm64 and current stable 
(6.13.4) when compiled with rustc 1.85.0:


```
   RUSTC L rust/kernel.o
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

For more information about this error, try `rustc --explain E0308`.
```


This is because rustc 1.85 (now stable) switched core::ffi::c_char from 
i8 to u8 on arm64 and other platforms [1], and because current stable 
still uses rustc's core's instead of the custom ffi integer types like 
6.14 will. Looking for other i8's in *.rs files tells me only the QR 
code panic screen should be affected in addition to the firmware 
abstractions, and that was already reported in [2].

A simple fix would be to switch i8 to u8 in `struct FwFunc`, but that 
breaks rustc <= 1.84 so I guess a more robust solution is needed.

Cheers!


[1] https://github.com/rust-lang/rust/pull/132975
[2] 
https://lore.kernel.org/all/20250120124531.2581448-1-linkmauve@linkmauve.fr/