Input: synaptics - replace NOOP with suitable commentary

Fixes the following W=1 kernel build warning(s):

 drivers/input/mouse/synaptics.c: In function ‘synaptics_process_packet’:
 drivers/input/mouse/synaptics.c:1110:6: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210114152323.2382283-2-lee.jones@linaro.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/synaptics.c

@@ -1106,8 +1106,11 @@ static void synaptics_process_packet(struct psmouse *psmouse)
 					num_fingers = hw.w + 2;
 				break;
 			case 2:
-				if (SYN_MODEL_PEN(info->model_id))
-					;   /* Nothing, treat a pen as a single finger */
+				/*
+				 * SYN_MODEL_PEN(info->model_id): even if
+				 * the device supports pen, we treat it as
+				 * a single finger.
+				 */
 				break;
 			case 4 ... 15:
 				if (SYN_CAP_PALMDETECT(info->capabilities))