ASoC: WM8903: Fix mic detection register definitions

* There is no hysteresis enable field in the current datasheet.
* Mic detection threshold field is only 2 bits wide.

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