ALSA: usb-audio: Change the semantics of the enable option

This patch changes the semantics of the enable option for snd-usb-audio
in order to allow users to disable a device specified by either or both
of the vendor id and the product id.

Signed-off-by: Tamaki Nishino <otamachan@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>