HID: wacom: Initialize hid_data.inputmode to -1

Commit 5ae6e89f introduced hid_data.inputmode with a comment that it
would have the value -1 if undefined, but then forgot to actually
perform the initialization. Although this doesn't appear to have
caused any problems in practice, it should still be remedied.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>