mfd: wm8994: Don't use system PM operations

For realistic systems the power management is controlled entirely via
runtime PM - if the device is not runtime suspended then the suspend
will abort without doing anything as functionality such as accessory
detection or audio bypass will require the device to be enabled while
if the device is runtime suspended it is already in the lowest power
state. This means that system suspend is redundant and can be removed
which avoids issues with attempting to double disable the regulators.

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