ASoC: arizona: Tidy up SYNCCLK selection and cache values

This patch caches the current SYNCCLK settings in the arizona_fll struct
and uses these to simplify the code which determines which source should
be used for the REFCLK and SYNCCLK inputs.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>