ALSA: usb-audio: Refactor clock finder helpers

There are lots of open-coded functions to find a clock source,
selector and multiplier.  Now there are both v2 and v3, so six
variants.

This patch refactors the code to use a common helper for the main
loop, and define each validator function for each target.
There is no functional change.

Fixes: 9a2fe9b8

 ("ALSA: usb: initial USB Audio Device Class 3.0 support")
Reviewed-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>