Input: wacom - process pen data in its own routine

So it would be easier for patch reviewers to follow the data path.

Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>