net/mlx4: Add SET_PORT opcode modifiers enumeration

The calls to SET_PORT used hard-code numbers, when supplying command's
opcode modifiers, fix that to use well defined constants.

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>