ASoC: fsl_ssi: Redefine RX and TX macros

The RX and TX macros were defined implicitly and there was
a potential risk if someone changes their values.

Since they were defined to index the array ssi->regvals[2],
this patch moves these two macros to fsl_ssi.c, closer to
its owner ssi->regvals. And it also puts some comments here
to limit their value within [0, 1].

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Caleb Crome <caleb@crome.org>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>