ALSA: wavefront: Support PCM sync_stop

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

Link: https://lore.kernel.org/r/20191210063454.31603-48-tiwai@suse.de


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