ALSA: usb-audio: Refactoring UAC2/3 clock setup code

This patch just does refactoring of the UAC2/3 clock setup code.
There should be no functional changes.  The major changes are:

* Provide union objects for pointing both UAC2 and UAC3 objects
* Unify clock source, selector and multiplier helper functions
* Unify __uac_clock_find_source() to deal with both UAC2 and UAC3
  equally

Link: https://lore.kernel.org/r/20210518152112.8016-2-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>