ASoC: fsl: create function imx_pcm_fiq_exit()

Create function imx_pcm_fiq_exit() to be paired with imx_pcm_fiq_init()
just like the pair of imx_pcm_dma_init() and imx_pcm_dma_exit().

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