net/mlx5: Set num_vhca_ports capability

Set the current capability to the max capability. Doing so enables dual
port RoCE functionality if supported by the firmware.

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>