When USB cable gets disconnected, the undergoing playback/capture
stalls, without any notification to u_audio about the change.
Experiments with a dwc2 gadget revealed that Suspend interrupt is
thrown at cable disconnection, which the gadget framework translates to
calling suspend callback of a function, if it is defined.

Add the suspend callback to f_uac2 function, calling
corresponding method of u_audio in order to stop the respective PCM
streams and to notify subscribed clients at cable disconnection.
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-10-pavel.hofman@ivitera.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>