usb: gadget: split out audio core

Abstract the peripheral side ALSA sound card code 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

This change differs from opensource patch in that it does not
modify the UAC2 driver to use u_audio helper file for ALSA
sound card support.

Change-Id: Ia67ec02ed442a67bc359efc2655aa9ba49b79368
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: eb9fecb9e69b0be8c267c55b0bb52a08e8fb6bee
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>