ASoC: fsl_ssi: Keep ssi->i2s_net updated

The hw_params() overwrites i2s_net settings for special cases like
mono-channel support, however, it doesn't update ssi->i2s_net as
set_dai_fmt() does.

This patch removes the local i2s_net variable and directly updates
ssi->i2s_net in the hw_params() so that the driver can simply look
up the ssi->i2s_net instead of reading the register.

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>