Input: wacom - use per-device instance of wacom_features

Since we mangle data in wacom_features when dealing with certain devices let's
use a private (per-device) instance of wacom_features in wacom_wac. This way
same product ID can support more than one type of device, such as pen and touch,
and not interfere with each other.

Signed-off-by: Jason Childs <oblivian@users.sourceforge.net>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>