dma_request_slave_channel() is a more appropriate API for dmaengine
clients that adopt generic DMA bindings to call.  Let's use it instead
of of_dma_request_slave_channel() to save <linux/of_dma.h> include.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>
Acked-by: NLars-Peter Clausen <lars@metafoo.de>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>