Input: wacom - remove code duplication

Replaces code to calculate Intuos5 physical dimensions with a call
to an existing function that performs the same task.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>