HID: wacom: Further clean up wacom_intuos_general packet decoder

Continue re-organizing and trimming cases to make it easier to wrap
the brain around. A number of changes were made after consulting the
protocol spec and so don't necessarily follow from the code itself.

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