mfd: Don't suspend WM8994 if the CODEC is not suspended

ASoC supports keeping the audio subsysetm active over suspend in order
to support use cases such as audio passthrough from a cellular modem
with the main CPU suspended. Ensure that we don't power down the CODEC
when this is happening by checking to see if VMID is up and skipping
suspend and resume when it is. If the CODEC has suspended then it'll
turn VMID off before the core suspend() gets called.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>