HID: wacom: Move Intuos pad handling code into dedicated function

Begin slimming down the body of 'wacom_intuos_irq' by moving out its
largest block of code to a dedicated 'wacom_intuos_pad' function.
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>