ALSA: usb-audio: update clock valid control

Make the "clock valid" control a global control instead of a mixer
so that it doesn't appear in mixer applications.

Additionally, remove the check for writeability prohibited by spec, and
Use common code to read the control value.

Tested with a UAC2 Audio device that presents a clock validity
control.  The control still shows up in /proc usbmixer but not
in alsamixer.

Signed-off-by: Andrew Chant <achant@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>