HID: wacom: Simplify check for presence of single-finger touch

To determine if a touch is present in the single-touch case, we can
simply check if the BTN_TOUCH key is active or not. This will work for
both HID_GENERIC and other device types.

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