driver:wm9705: Check and re-set dig2 config register

Due to an unknown reason, the dig2 config register resets randomly. This
disables the generation of pen-down interrupts. Without the interrupt, doesn't
sample further touches.
As a first workaround, we check the dig2 config register before exiting the
sample function and re-set the config value, if it was reset.

BCS 746-001452
BCS 746-001447

drivers/input/touchscreen/wm9705.c

@@ -232,6 +232,7 @@ static int wm9705_poll_sample(struct wm97xx *wm, int adcsel, int *sample)
 	int valid_samples = 0;
 	int flags = 0;
 	int ret;
+	u16 dig2 = 0;
 
 	if (wants_pen && !wm->pen_probably_down) {
 		u16 data = wm97xx_reg_read(wm, AC97_WM97XX_DIGITISER_RD);
@@ -276,6 +277,10 @@ static int wm9705_poll_sample(struct wm97xx *wm, int adcsel, int *sample)
 
 		wm97xx_reg_write(wm, AC97_WM97XX_DIGITISER1, 0);
 
+		dig2 = wm97xx_reg_read(wm, AC97_WM97XX_DIGITISER2);
+		if (dig2 == 0)
+			wm97xx_reg_write(wm, AC97_WM97XX_DIGITISER2, wm->dig[2]);
+
 		flags = *sample & 0xF000;
 
 		dev_dbg(wm->dev, "u(%u): %04u %04u %04u %04u %04u %04u %04u %04u %04u %04u => %04u", adcsel,