ASoC: codec: wm9860: Refactor PLL out freq search

Add a separate function for deriving (sysclk, lrclk, bclk)
when the clock is auto or pll.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>