usb: gadget: u_audio: Add suspend call

Add exported method u_audio_suspend which sets stream status to
inactive and sends notifications. The method does not free any
resources.

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-9-pavel.hofman@ivitera.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>