IB/mlx5: Reduce the use of num_port capability

Remove use of the num_ports general capability throughout. The number of
ports will be variable in the future, and reported in a different way.

Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>