ALSA: usb-audio: Add resume support for FTU controls

A few FTU mixer controls have the own value handling, so they have to
be rewritten to follow the support for resume callbacks.  This ended
up in a fair amount of refactoring.  Its own struct is now removed,
instead the values are embedded in kctl private_value totally.

Signed-off-by: Takashi Iwai <tiwai@suse.de>