iio: adc: at91-sama5d2_adc: ack DRDY irq in direct mode

Need to acknowledge DRDY irq in direct mode/ software
triggered mode. Otherwise, on the next conversion, overrun
flag will be raised, which is not a correct state.
This doesn't affect the functionality, but will generate
possible incorrect overrun reports.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>