net: realtek: Remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/realtek/8139cp.c: In function cp_tx_timeout:
drivers/net/ethernet/realtek/8139cp.c:1242:6: warning: variable ‘rc’ set but not used [-Wunused-but-set-variable]

`rc` is never used, so remove it.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>