mfd: ti_am335x_tscadc: Fix TSC operation after ADC continouous mode

After enabling and disabling ADC continuous mode via sysfs, ts_print_raw
fails to return any data. This is because when ADC is configured for
continuous mode, it disables touch screen steps.These steps are not
re-enabled when ADC continuous mode is disabled. Therefore existing values
of REG_SE needs to be cached before enabling continuous mode and
disabling touch screen steps and enabling ADC steps. The cached value
are to be restored to REG_SE once ADC is disabled.

Fixes: 7ca6740c ("mfd: input: iio: ti_amm335x: Rework TSC/ADC synchronization")

Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/ti_am335x_tscadc.c

@@ -53,7 +53,7 @@ void am335x_tsc_se_set_cache(struct ti_tscadc_dev *tsadc, u32 val)
 	unsigned long flags;
 
 	spin_lock_irqsave(&tsadc->reg_lock, flags);
-	tsadc->reg_se_cache = val;
+	tsadc->reg_se_cache |= val;
 	if (tsadc->adc_waiting)
 		wake_up(&tsadc->reg_se_wait);
 	else if (!tsadc->adc_in_use)
@@ -96,6 +96,7 @@ static void am335x_tscadc_need_adc(struct ti_tscadc_dev *tsadc)
 void am335x_tsc_se_set_once(struct ti_tscadc_dev *tsadc, u32 val)
 {
 	spin_lock_irq(&tsadc->reg_lock);
+	tsadc->reg_se_cache |= val;
 	am335x_tscadc_need_adc(tsadc);
 
 	tscadc_writel(tsadc, REG_SE, val);