dma/imx-sdma: call sdma_set_channel_priority after sdma_request_channel

sdma_request_channel sets the default priority. sdma_alloc_chan_resources
should call sdma_set_channel_priority thereafter to over write it.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>