devlink: Prevent port_type_set() callback when it's not needed

When a port_type_set() is been called and the new port type set is the same
as the old one, just return success.

Signed-off-by: Elad Raz <eladr@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>