ASoC: fsl_ssi: Replace fsl_ssi_rxtx_reg_val with fsl_ssi_regvals

The name fsl_ssi_rxtx_reg_val is too long to read comfortably.
So this patch shortens it by using an array (fsl_ssi_regvals,
renamed from fsl_ssi_reg_val). To do that, it also introduces
two macros (TX and RX) to replace the wrapper structure. This
will also help further cleanups.

Meanwhile, it unifies all local variable with the name "vals"
to get rid of the name "reg" -- could be confusing with "regs"
in the private struct for regmap.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Mark Brown <broonie@kernel.org>