cxgb4: remove set but not used variable 'tab'

drivers/net/ethernet/chelsio/cxgb4/cxgb4_filter.c: In function cxgb4_get_free_ftid:
drivers/net/ethernet/chelsio/cxgb4/cxgb4_filter.c:547:23:
 warning: variable tab set but not used [-Wunused-but-set-variable]

commit 8d174351 ("cxgb4: rework TC filter rule insertion across regions")
involved this, remove it.

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