ALSA: ASoC: Implement WM8510 bias level control

The WM8510 bias level configuration blindly overwrites the power
management registers, interfering with the operation of DAPM.
Only adjust the specific bits required, implementing use of the VMID
resistor string configuration control as we go.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>