HID: wacom: Correct base usage for capacitive ExpressKey status bits

The capacitive status of ExpressKeys is reported with usages beginning
at 0x940, not 0x950. Bring our driver into alignment with reality.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>