devlink: Move switch_port attribute of devlink_port_attrs to devlink_port

The struct devlink_port_attrs holds the attributes of devlink_port.

Similarly to the previous patch, 'switch_port' attribute is another
exception.

Move 'switch_port' to be devlink_port's field.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>