iio: adc: at91-sama5d2_adc: Use dma_request_chan() instead dma_request_slave_channel()

iio: adc: at91-sama5d2_adc: Use dma_request_chan() instead dma_request_slave_channel()

dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

The dma_request_chan() is the standard API to request slave channel,
clients should be moved away from the legacy API to allow us to retire
them.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>