IB/mlx4: Handle FW command interface rev 3

Upcoming firmware introduces command interface revision 3, which
changes the way port capabilities are queried and set.  Update the
driver to handle both the new and old command interfaces by adding a
new MLX4_FLAG_OLD_PORT_CMDS that it is set after querying the firmware
interface revision and then using the correct interface based on the
setting of the flag.

Signed-off-by: Roland Dreier <rolandd@cisco.com>