cxgb4: silence shift wrapping static checker warning

I don't know how large "tp->vlan_shift" is but static checkers worry
about shift wrapping bugs here.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>