net/mlx4_core: Remove unnecessary validation for port number

This is a fix to a regression introduced by commit:
"982290a7 net/mlx4_core: Check port number for validity
before accessing data"

IPoIB could not attach to multicast group and we get this in dmesg:
[144214.145008] ib0: failed to attach to multicast group, ret = -22
[144214.145016] ib0: couldn't attach QP to multicast group ff12:401b:ffff:0000:0000:0000:ffff:ffff
[144214.145019] ib0: multicast join failed for ff12:401b:ffff:0000:0000:0000:ffff:ffff, status -22

The cause to the problem is because port is extracted from gid[5].
Which is only valid for Ethernet.
Removed this validation in mlx4_qp_attach_common(), which is accessed
from both Ethernet and IB flows.
Error flow for bad port value in Ethernet is already exists in that
function.

Signed-off-by: Moni Shoua <monis@mellanox.co.il>
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>