IB/mlx4: Take source mac from AH instead from the port

In commit dbf727de ("IB/core: Use GID table in AH creation and dmac
resolution") we copy source mac to mlx4_ah from the attributes of
gid at ib_ah_attr.grh.sgid_index. Now we can use it.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>