mmc: owl-mmc: 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.

By using dma_request_chan() directly the driver can support deferred
probing against DMA.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191217112934.31535-1-peter.ujfalusi@ti.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>