ASoC: fsl: correct get_dma_channel parameter name

The second parameter of function get_dma_channel is actually a property
name rather than a compatible string, so rename it for less confusing.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>