dma/imx-sdma: move clk_enable out of sdma_request_channel

It makes clk_enable/disable pair more readable, and fix one bug:
sdma_init calls sdma_request_channel, but seems don't know
sdma_request_channel enabled the clock.

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
[fixed typo in commit log]
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>