spi: stm32: Use dma_request_chan() instead dma_request_slave_channel()

spi: stm32: Use dma_request_chan() instead dma_request_slave_channel()

[ Upstream commit 0a454258febb73e4c60d7f5d9a02d1a8c64fdfb8 ]

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/20191212135550.4634-10-peter.ujfalusi@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>