net: pse-pd: Remove is_enabled callback from drivers

The is_enabled callback is now redundant as the admin_state can be obtained
directly from the driver and provides the same information.

To simplify functionality, the core will handle this internally, making
the is_enabled callback unnecessary at the driver level. Remove the
callback from all drivers.

Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>