firmware: fix detecting error on register_reboot_notifier()

register_reboot_notifier() can fail, detect this and address this
failure. This has been broken since v3.11, however the chances of
this failing here is really low.

Fixes: fe304143 ("firmware: Avoid deadlock of usermodehelper lock at shutdown")
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>