net/mlx4_core: Check port number for validity before accessing data

Need to validate port number at mlx4_promisc_qp() before use.
Since port number is extracted from gid, as a cooked or corrupted gid
could lead to a crash.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>