iio: adc: ti_am335x_adc: Disable ADC during suspend unconditionally

Parent MFD device takes care of enabling ADC interface whenever
touchscreen is marked wakeup capable. Therefore, unconditionally disable
ADC interface during system suspend to save power in case of system with
just ADC and no TSC.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>