HID: wacom: Slim down wacom_intuos_pad processing

Seperate the function into two halves: first gather data from the packet,
next report all gathered data. The input subsystem should automatically
mute any events that aren't actually declared for the tablet at hand.

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