HID: wacom: Add speed setting for Intuos4 WL

Add option to change reporting speed for Intuos4 WL. The option is only
internal to the module, but it will be extended to allow control over sysfs,
as it is already implemented for Graphire.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>