ALSA: usb-audio: add UAC2 clock sources as mixer controls

UAC2 specifies clock sources that optionally have validity controls.
This patch exposes them as mixer controls, so they can be read (and
at least in theory even be written) by userspace applications in order
to make clock selection policy decisions.

This implementation does nothing if the device is not UAC2 compliant,
or if the clock source does not define said validity control bits.

Tested with a miniDSP USBStreamer (0x2752/0x0016).

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>