ASoC: generic-dmaengine-pcm: call dma_request_slave_channel()

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: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>