HID: wacom: Support touchrings with relative motion

If a touchring is configured to send relative events (e.g. +1 or -1 every
time some bit of rotational distance is covered), we should similarly
send relative events up to userspace. Previous non-HID tablets used
REL_WHEEL to send this kind of information, so we opt to use this same
axis since userspace (xf86-input-wacom and libinput) already expects
this kind of behavior from the Wacom kernel driver.

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