The comment in the remove callback suggests that the driver is not
supposed to be unbound. However returning an error code in the remove
callback doesn't accomplish that. Instead set the suppress_bind_attrs
property (which makes it impossible to unbind the driver via sysfs).
The only remaining way to unbind a em_sti device would be module
unloading, but that doesn't apply here, as the driver cannot be built as
a module.

Also drop the useless remove callback.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230207193010.469495-1-u.kleine-koenig@pengutronix.deSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>