ASoC: fsl_ssi: Rename fsl_ssi_private to fsl_ssi

Shorten the private data structure to save some wrapped lines.
Signed-off-by: NNicolin Chen <nicoleotsuka@gmail.com>
Tested-by: NMaciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: NMaciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: NTimur Tabi <timur@tabi.org>
Signed-off-by: NMark Brown <broonie@kernel.org>