ASoC: rsnd: check src mod pointer for rsnd_mod_id()

Without this patch, gcc 4.9.x says

    sound/soc/sh/rcar/cmd.c: In function 'rsnd_cmd_init':
    sound/soc/sh/rcar/cmd.c:85:14: warning: array subscript is below array\
	bounds [-Warray-bounds]
	data = path[rsnd_mod_id(src)] |

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