HID: wacom: power_supply: provide the actual model_name

Instead of displaying a generic "tablet", now g-c-c shows a pretty
"Wacom Intuos Pro S (WL)".

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>