ALSA: usb-mixer: factor out quirks

Move all non-standard mixer controls and vendor-specific extensions to a
separate file. Some structs need to be exported now.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>