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

drivers/net/ethernet/intel/e1000/e1000_main.c: In function 'e1000_watchdog':
drivers/net/ethernet/intel/e1000/e1000_main.c:2436:9: warning:
 variable 'txb2b' set but not used [-Wunused-but-set-variable]
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>