ASoC: wm8960: Let wm8960 codec driver manage its own MCLK

When we want to use wm8960 codec, we should enable its MCLK in machine driver.
It's reasonable for wm8960 codec driver to manage its own MCLK.

When current bias_level is SND_SOC_BIAS_ON, it is preparing for a transition
away from ON. In this case, disable the codec mclk. When current bias_level
is not SND_SOC_BIAS_ON, it preparing for a transition to ON. In this case,
enable the codec mclk.

Signed-off-by: Zidan Wang <b50113@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>