ALSA: pcm: optimize xrun detection in no-period-wakeup mode

Add a lightweight condition on top of the xrun checking so that we can
avoid the division when the application is calling the update function
often enough.

Suggested-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>