HID: wacom: generic: support LEDs

Add support for the LEDs around the mode switch to the generic code path in
support of the second generation Intuos Pro.

Signed-off-by: Aaron Skomra <aaron.skomra@wacom.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>