This patch fixes this WARNING

sound/soc/sh/rcar/ssi.c:285:5-14: WARNING: Unsigned expression\
	compared with zero: main_rate < 0
Signed-off-by: NKuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: NMark Brown <broonie@kernel.org>