greybus: audio-pcm: fix use of variable unitialized

Compiling with clang shows that period_elapsed will be used as a branch
conditional unitialized whenever snd_dev->transfer_done <
runtime->period_size. Since stack can grow up/down as we proceed though
the call stack this should be fixed. This patch fixes by explicitly
initalizing period_elapsed to zero.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/audio-pcm.c

@@ -91,6 +91,7 @@ static void gb_pcm_work(struct work_struct *work)
 
 	frames = (len + (oldptr % stride)) / stride;
 
+	period_elapsed = 0;
 	snd_dev->transfer_done += frames;
 	if (snd_dev->transfer_done >= runtime->period_size) {
 		snd_dev->transfer_done -= runtime->period_size;