ASoC: fsl_ssi: Maintain a mask of active streams

Checking TE and RE bits in SCR register doesn't work for AC97 mode
which enables SSIEN, TE and RE in the fsl_ssi_setup_ac97() that's
called during probe().

So when running into the trigger(), it will always get the result
of both TE and RE being enabled already, even if actually there is
no active stream.

This patch fixes this issue by adding a variable to log the active
streams manually.

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>