asus_acpi: fix oops on non-asus machines

If asus_acpi_init doesn't find any device it knows about, it mistakenly
returns a "success" error code even though it cleans up after itself. Later
when trying to rmmod asus_acpi, the module_exit routine would try to clean up
one more time and we would end up calling
acpi_bus_unregister_driver(&asus_hotk_driver) twice. This patch addresses
this first problem by returning -ENODEV when no appropriate device is found.

Then there was also another bug with the code handling the return value of
backlight_device_register. If this function ever failed, the driver would
cleanup by calling the module_exit routine from module_init, but it would
still return "success". So any attempt to rmmod this module would result in
asus_acpi_exit being called twice but it's not ready to handle it (I haven't
hit this bug, just found it by code inspection). This patch fixes that by
inserting a return -ENODEV; at the end of this error handling path.

Signed-off-by: Maxime Austruy <maxime@tralhalla.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>