devlink: Constify devlink instance pointer

Constify devlink instance pointer while checking if reload operation is
supported or not.

This helps to review the scope of checks done in reload.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>