mfd: wm8994: Make current device behaviour the default

As the wm8994 series of devices are now very mature make the current
behaviour of the devices the default behaviour, any future revisions are
likely to have only minor updates.

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