Input: resistive-adc-touch - fix division by zero error on z1 == 0

For proper pressure calculation we need at least x and z1 to be non
zero. Even worse, in case z1 we may run in to division by zero
error.

Fixes: 60b7db91 ("Input: resistive-adc-touch - rework mapping of channels")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20211007095727.29579-1-o.rempel@pengutronix.deSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/resistive-adc-touch.c

@@ -71,6 +71,7 @@ static int grts_cb(const void *data, void *private)
 		unsigned int z2 = touch_info[st->ch_map[GRTS_CH_Z2]];
 		unsigned int Rt;
 
+		if (likely(x && z1)) {
 			Rt = z2;
 			Rt -= z1;
 			Rt *= st->x_plate_ohms;
@@ -79,12 +80,14 @@ static int grts_cb(const void *data, void *private)
 			Rt /= z1;
 			Rt = DIV_ROUND_CLOSEST(Rt, 256);
 			/*
-		 * On increased pressure the resistance (Rt) is decreasing
-		 * so, convert values to make it looks as real pressure.
+			 * On increased pressure the resistance (Rt) is
+			 * decreasing so, convert values to make it looks as
+			 * real pressure.
 			 */
 			if (Rt < GRTS_DEFAULT_PRESSURE_MAX)
 				press = GRTS_DEFAULT_PRESSURE_MAX - Rt;
 		}
+	}
 
 	if ((!x && !y) || (st->pressure && (press < st->pressure_min))) {
 		/* report end of touch */