dmaengine: sprd: Optimize the sprd_dma_prep_dma_memcpy()

This is one preparation patch, we can use default DMA configuration to
implement the device_prep_dma_memcpy() interface instead of issuing
sprd_dma_config().

We will implement one new sprd_dma_config() function with introducing
device_prep_slave_sg() interface in following patch. So we can remove
the obsolete sprd_dma_config() firstly.

Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>