ALSA: vxpocket: Support PCM sync_stop

The driver invokes snd_pcm_period_elapsed() simply from the threaded
interrupt handler.  Set card->sync_irq for enabling the missing
sync_stop PCM operation.

Link: https://lore.kernel.org/r/20191210063454.31603-55-tiwai@suse.de


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