ASoC: arizona: Improve handling of setting REFCLK to 0

This patch suppresses calculation of REFCLK parameters when the REFCLK
source frequency is set to zero, additionally it will consider a source
frequency of zero as the REFCLK being disabled and switch to using the
SYNCCLK.

Reported-by: Kyung Kwee Ryu <Kyung-Kwee.Ryu@wolfsonmicro.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>