usb: gadget: u_audio: add bi-directional volume and mute support

USB Audio Class 1/2 have ability to change device's
volume and mute by USB Host through class-specific control
requests. Device also can notify Host about volume/mute
change on its side through optional interrupt endpoint.

This patch adds Volume and Mute ALSA controls which can be
used by user to send and receive notifications to/from
the USB Host about Volume and Mute change.

These params come from f_uac* so volume and mute controls
will be created only if the function support and enable
each explicitly

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20210712125529.76070-3-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>