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: NParav Pandit <parav@mellanox.com>
Reviewed-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>