net/mlx5: Support enable_rdma devlink dev param

Enable user to disable RDMA auxiliary device so that when it is not
required, user can disable it.

For example,

$ devlink dev param set pci/0000:06:00.0 \
              name enable_rdma value false cmode driverinit
$ devlink dev reload pci/0000:06:00.0

At this point devlink instance do not create auxiliary device
mlx5_core.rdma.2 for the RDMA functionality.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>