spi: s3c64xx: Don't request/release DMA channels for each SPI transfer

Requesting a DMA channel might be a time consuming operation, so there is
no need to acquire and release DMA channel for each SPI transfer.
DMA channels can be requested during driver probe and kept all the time,
also because there are no shared nor dynamically allocated channels on
Samsung S3C/S5P/Exynos platforms.

While moving dma_requrest_slave_channel calls, lets switch to
dma_request_slave_channel_reason(), which returns error codes on failure,
which can be properly propagated to the caller (this for example defers
SPI probe when DMA controller is not yet available).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Tested-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>