ASoC: fsl_ssi: Use ssi->streams instead of reading register

Since ssi->streams is being updated along with SCR register and
its SSIEN bit, it's simpler to use it instead.

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>