Instead of resorting to BUG() ensure that the driver isn't unbound by
suppressing its bind and unbind sysfs attributes. As the driver is
built-in there is no way to remove a device once bound.

As a nice side effect this allows to drop the remove function.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230726081442.461026-1-u.kleine-koenig@pengutronix.de