mfd: Add more checks for WM8994 being active during suspend

Enhancements to the WM8994 audio driver and new features on more modern
devices in the series mean that we can no longer rely on VMID being active
as an indication that the device is active. Add further checks for digital
paths and microphone detection.

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