ASoC: rsnd: rsnd_ssi_run_mods() needs to care ssi_parent_mod

SSI parent mod might be NULL. ssi_parent_mod() needs to care
about it. Otherwise, it uses negative shift.
This patch fixes it.

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