ASoC: wm8994: Bring WM8994 accessory detection up to date

Make the mechanism used for WM8994 more like that for WM1811 and WM8958:
provide the logic to distinguish between headphone and headset and hard
code the reporting of sensible SND_JACK values. Should integration with
other detection mechanisms be required we can add appropriate callbacks
(though some integrations should be able to use the subsystem ones).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>