HID: wacom: Move parsing to a separate function

This patch doesn't change the way driver works. Parsing logic is now in a
separate function. It's a first step to add Intuos4 Wireless support to
hid-wacom driver.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Acked-by: Ping Cheng <pinglinux@gmail.com>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>