[PATCH v2] Fix tulip breakage

[Helge Deller <deller@gmx.de>]

The tulip network driver in a qemu-system-hppa emulation is broken in
the sense that bigger network packages aren't received any longer and
thus even running e.g. "apt update" inside the VM fails.

The breakage was introduced by commit 8ffb7265af ("check frame size and
r/w data length") which added checks to prevent accesses outside of the
rx/tx buffers.

But the new checks were implemented wrong. The variable rx_frame_len
counts backwards, from rx_frame_size down to zero, and the variable len
is never bigger than rx_frame_len, so accesses just can't happen and the
checks are unnecessary.
On the contrary the checks now prevented bigger packages to be moved
into the rx buffers.

This patch reverts the wrong checks and were sucessfully tested with a
qemu-system-hppa emulation.

Fixes: 8ffb7265af ("check frame size and r/w data length")
Buglink: https://bugs.launchpad.net/bugs/1874539
Signed-off-by: Helge Deller <deller@gmx.de>

---
changes v2 to v1:
    - renamed patch title
    - rephrased commit message
    - no changes to patch itself

diff --git a/hw/net/tulip.c b/hw/net/tulip.c
index 1295f51d07..59d21defcc 100644
--- a/hw/net/tulip.c
+++ b/hw/net/tulip.c
@@ -171,9 +171,6 @@ static void tulip_copy_rx_bytes(TULIPState *s, struct tulip_descriptor *desc)
             len = s->rx_frame_len;
         }

-        if (s->rx_frame_len + len > sizeof(s->rx_frame)) {
-            return;
-        }
         pci_dma_write(&s->dev, desc->buf_addr1, s->rx_frame +
             (s->rx_frame_size - s->rx_frame_len), len);
         s->rx_frame_len -= len;
@@ -186,9 +183,6 @@ static void tulip_copy_rx_bytes(TULIPState *s, struct tulip_descriptor *desc)
             len = s->rx_frame_len;
         }

-        if (s->rx_frame_len + len > sizeof(s->rx_frame)) {
-            return;
-        }
         pci_dma_write(&s->dev, desc->buf_addr2, s->rx_frame +
             (s->rx_frame_size - s->rx_frame_len), len);
         s->rx_frame_len -= len;