ASoC: fsi: tidyup sh_fsi_platform_info pointer

Current FSI driver is requesting sh_fsi_platform_info pointer from platform,
and it didn't allowed NULL pointer.
This patch fixes it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>