e1000: remove set but not used variable 'txb2b'

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: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>