ASoC: sh_fsi: avoid using global variable

Current FSI driver use global variable to access device data.
But this style will be broken
if SuperH come with multiple FSI blocks in future.
To solve this problem, this patch use cpu_dai->private_data.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>