spi: spi-s3c64xx: Check return values

Check return values in prepare_dma() and s3c64xx_spi_config() and
propagate errors upwards.

Fixes: 78843727 ("spi: s3c64xx: move to generic dmaengine API")
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Link: https://lore.kernel.org/r/20201002122243.26849-4-l.stelmach@samsung.com


Signed-off-by: Mark Brown <broonie@kernel.org>