If mic detect is left enabled and the WM8903 detects a status change,
the WM8903 driver will make a callback against the free()d jack, which
will cause a crash.

This problem can be triggered by fully initializing an audio card, then
removing and re-inserting the machine driver module.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>