mlx4_core: For RoCE, allow slaves to set the GID entry at that slave's index

For IB transport, the host determines the slave GIDs. For ETH (RoCE),
however, the slave's GID is determined by the IP address that the slave
itself assigns to the ETH device used by RoCE.

In this case, the slave must be able to write its GIDs to the HCA gid table
(at the GID indices that slave "owns").

This commit adds processing for the SET_PORT_GID_TABLE opcode modifier
for the SET_PORT command wrapper (so that slaves may modify their GIDS
for RoCE).

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>