dmaengine: imx-sdma: remove dma_slave_config direction usage and leave sdma_event_enable()

dma_slave_config direction was marked as deprecated quite some
time back, remove the usage from this driver so that the field
can be removed

ENBLn bit should be set before any dma request triggered, please
refer to the below information from i.mx6sololite RM. Otherwise,
spi/uart test will be fail because there is dma request from tx
fifo always before dmaengine_prep_slave_sg() in where ENBLn set
and violate the below rule.

https://www.nxp.com/docs/en/reference-manual/IMX6SLRM.pdf:

40.8.28 Channel Enable RAM (SDMAARM_CHNENBLn)
"It is thus essential for the Arm platform to program them before
any DMA request is triggered to the SDMA, otherwise an unpredictable
combination of channels may be started".

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
[vkoul: sqashed patch from Robin into direction change]
Signed-off-by: Vinod Koul <vkoul@kernel.org>