alsa-restore.rules: refer to correct attr

$attr{number} in the RUN rule is an empty expansion. This makes sense,
because the path doesn't exist -- i.e., it refers to the path:

/sys/devices/pci0000:00/foo/bar/sound/card0/controlC0/number

Instead, refer to $attr{device/number}, which does exist.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>