ASoC: arizona: Do not test ratio zero as it is not a valid setting

Zero is not a valid FRATIO for the FLL, as such we shouldn't test it
whilst refining the FRATIO. This patch does just that.

Reported-by: Ryo Tsutsui <ryo.tsutsui@wolfsonmicro.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>