Input: ad7879 - fix deficient device disable

Input close or device disable should not interact with the exported
gpiolib functionality. However that's the case. __ad7879_disable()
clears the entire AD7879_REG_CTRL2, while it should just power down
the ADC and its reference.
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/touchscreen/ad7879.c

@@ -249,12 +249,14 @@ static void __ad7879_enable(struct ad7879 *ts)
 
 static void __ad7879_disable(struct ad7879 *ts)
 {
+	u16 reg = (ts->cmd_crtl2 & ~AD7879_PM(-1)) |
+		AD7879_PM(AD7879_PM_SHUTDOWN);
 	disable_irq(ts->irq);
 
 	if (del_timer_sync(&ts->timer))
 		ad7879_ts_event_release(ts);
 
-	ad7879_write(ts, AD7879_REG_CTRL2, AD7879_PM(AD7879_PM_SHUTDOWN));
+	ad7879_write(ts, AD7879_REG_CTRL2, reg);
 }