ASoC: fsl: let fsl_ssi work with imx pcm and machine drivers

Makes necessary changes on fsl_ssi to let it work with imx pcm and
machine drivers.

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