ALSA: usb-audio: Add support for Selector Units in UAC3

This patch add support for Selector Units and Clock Selector Units
defined in the new UAC3 spec.

Selector Units play a really important role in the new UAC3 spec as
Processing Units do not define an on/off switch control anymore.
This forces topology designers to add bypass paths in the topology
to enable/dissable the Processing Units.

Signed-off-by: Jorge Sanjuan <jorge.sanjuan@codethink.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>