ASoC: fsl-ssi: Remove unnecessary variables from ssi_private

There are some variables defined in struct fsl_ssi_private that describe
states that are also described by other variables.

This patch adds some helper functions that return exactly the same
information based on available variables. This helps to clean up struct
fsl_ssi_private and remove them from the probe function.

It also removes some not really used variables (new_binding, name).

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-By: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>