greybus: loopback: masked out threads should sleep

If a thread is masked out it should not consume CPU cycles during a test.
We set an arbitrary 100 millisecond sleep time for each masked out thread.
Reasonably blunt instrument to ensure threads with nothing to do don't end
up thrashing the acquisition/release of mutexes.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Patrick Titiano <ptitiano@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/loopback.c

@@ -693,8 +693,10 @@ static int gb_loopback_fn(void *data)
 			break;
 
 		mutex_lock(&gb_dev.mutex);
-		if (!gb_loopback_active(gb))
+		if (!gb_loopback_active(gb)) {
+			ms_wait = 100;
 			goto unlock_continue;
+		}
 		if (gb_dev.iteration_max) {
 			/* Determine overall lowest count */
 			low_count = gb->iteration_count;