rtl8139: avoid clobbering tx descriptor bits

rtl8139: avoid clobbering tx descriptor bits

The device turns the Tx Descriptor into a Tx Status descriptor after
fully reading the descriptor. This involves clearing Tx Own (bit 31) to
indicate that the driver has ownership of the descriptor again as well
as several other bits.

The code keeps the first dword of the Tx Descriptor in the txdw0 local
variable. txdw0 is reused to build the first word of the Tx Status
descriptor. Later on the code uses txdw0 again, incorrectly assuming
that it still contains the first dword of the Tx Descriptor. The tx
offloading code misbehaves because it sees bogus bits in txdw0.

Use a separate local variable for Tx Status and preserve Tx Descriptor
in txdw0.

Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221117165554.1773409-2-stefanha@redhat.com>