BACKPORT: usb: gadget: f_uac2: add volume and mute support

This adds bi-directional (host->device, device->host)
volume/mute support to the f_uac2 driver by adding
Feature Units and interrupt endpoint.

Currently only master channel is supported.

Volume and mute are configurable through configfs,
by default volume has -100..0 dB range with 1 dB step.

Similar to existing flexible endpoints configuration,
Feature Unit won't be added to the topology if both
mute and volume are not enabled, also interrupt endpoint
isn't added to the device if no feature unit is present

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20210712125529.76070-4-pavel.hofman@ivitera.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Bug: 235196743
(cherry picked from commit eaf6cbe0)
[luizmmat: Resolved minor conflict in f_uac2.c]
Change-Id: Ie8e0de9ac43d2d2030e530e8b244760f45627d7e
Signed-off-by: Luiz Matheus <luizmmat@motorola.com>