Input: ti_am335x_tsc - fix STEPCONFIG setup for Z2

The Z2 step configuration doesn't erase the SEL_INP_SWC_3_0 bit-field
before setting the ADC channel. This way its value could be corrupted by
the ADC channel selected for the Z1 coordinate.

Fixes: 8c896308 ("input: ti_am335x_adc: use only FIFO0 and clean up a little")
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Link: https://lore.kernel.org/r/20211212125358.14416-3-dariobin@libero.itSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/ti_am335x_tsc.c

@@ -196,7 +196,10 @@ static void titsc_step_config(struct titsc *ts_dev)
 			STEPCONFIG_OPENDLY);
 
 	end_step++;
-	config |= STEPCONFIG_INP(ts_dev->inp_yn);
+	config = STEPCONFIG_MODE_HWSYNC |
+			STEPCONFIG_AVG_16 | ts_dev->bit_yp |
+			ts_dev->bit_xn | STEPCONFIG_INM_ADCREFM |
+			STEPCONFIG_INP(ts_dev->inp_yn);
 	titsc_writel(ts_dev, REG_STEPCONFIG(end_step), config);
 	titsc_writel(ts_dev, REG_STEPDELAY(end_step),
 			STEPCONFIG_OPENDLY);