ALSA: usb-audio: replace /proc/bus/usb by /dev/bus/usb

The /proc/bus/usb devices don't exist anymore, since when we
got rid of usbfs. Those devices are now seen at
/dev/bus/usb.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>