ASoC: generic-dmaengine-pcm: use a more common dma name

The examples in Documentation/devicetree/bindings/dma/dma.txt recommends
the name for dma channel doing both RX and TX to be "rx-tx".  This
becomes a common pattern that has been adopted by platforms that
converts to generic DMA bindings.  Let's follow this common pattern in
generic-dmaengine-pcm.

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>