sound: Correct data output in sound_create_square_wave()

This function currently outputs twice as much data as it should and
overwrites its buffer as a result. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/sound/sound.c b/drivers/sound/sound.c
index 9694081..6c1eb4c 100644
--- a/drivers/sound/sound.c
+++ b/drivers/sound/sound.c
@@ -25,12 +25,10 @@
 		for (i = 0; size && i < half; i++) {
 			size -= 2;
 			*data++ = amplitude;
-			*data++ = amplitude;
 		}
 		for (i = 0; size && i < period - half; i++) {
 			size -= 2;
 			*data++ = -amplitude;
-			*data++ = -amplitude;
 		}
 	}
 }