ACPI: APEI: EINJ: mark remove callback as __exit

The einj_driver driver is registered using platform_driver_probe(). In
this case it cannot get unbound via sysfs and it's ok to put the remove
callback into an exit section. To prevent the modpost warning about
einj_driver referencing .exit.text, mark the driver struct with
__refdata and explain the situation in a comment.

This is an improvement over commit a24118a8 ("ACPI: APEI: EINJ: mark
remove callback as non-__exit") which recently addressed the same issue,
but picked a less optimal variant.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>