ide: unregister idepnp driver on unload

idepnp driver is registered as a pnp driver on ide init but doesn't
get unregistered on ide unload causing driver list corruption and
eventually oops.  Fix it.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>