ASoC: Simplify default WM8958 jack detection code

The default WM8958 jack detection handler implements a full set of buttons
and also support for video detection. Support for multi-button jacks is
fairly system specific and will usually require some tuning for headsets
so simplify the implementation to only report a simple short to ground
button, leaving multi-button headsets to be handled by system specific
code.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>