iio: adc: at91-sama5d2_adc: exit from write_raw() when buffers are enabled

iio: adc: at91-sama5d2_adc: exit from write_raw() when buffers are enabled

When buffers are enabled conversion may start asynchronously thus
allowing changes on actual hardware could lead to bad behavior. Thus
do not allow changing oversampling ratio and sample frequency when
if iio_device_claim_direct_mode() returns with error.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220803102855.2191070-6-claudiu.beznea@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>