Input: wacom - add fuzz parameters to features

The signal-to-noise ratio varies between devices, but currently all
devices are treated the same way. Add fuzz parameters to the feature
struct, allowing for tailored treatment of devices.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>