usb: gadget: f_uac2: split out audio core

Abstract the peripheral side ALSA sound card code from
the f_uac2 function into a component that can be called
by various functions, so the various flavors can be split
apart and selectively reused.

Visible changes:
 - add uac_params structure to pass audio paramteres for
   g_audio_setup
 - make ALSA sound card's name configurable
 - add [in/out]_ep_maxpsize
 - allocate snd_uac_chip structure during g_audio_setup
 - add u_audio_[start/stop]_[capture/playback] functions

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>