ASoC: rsnd: Fixup SRCm_IFSVR calculate method

This patch fixes the calculation accuracy degradation of SRCm_IFSVR
register value.

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