ALSA: usb-audio: Support PCM sync_stop

USB-audio driver had some implementation of its own sync-stop
mechanism.  This patch moved a part of it to the common PCM sync_stop
ops.

Link: https://lore.kernel.org/r/20191210063454.31603-56-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>