ASoC: wm8994: Enabling VMID should take a runtime PM reference

commit db966f8a upstream.

We can enable VMID independently of the bias in some use cases so we need
to ensure that the core device is powered up.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>