HID: wacom: Centralize Intuos pen packet decoding

Continue to slim down 'wacom_intuos_irq' by moving all decoding and
reporting of pen packet data into the  'wacom_intuos_general' function.

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