net/mlx4: Refactor QUERY_PORT

Currently QUERY_PORT is done as a part of QUERY_DEV_CAP firmware command.

Since we would like to use it without querying all device capabilities,
extract this part to be a function of its own.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>