Input: wacom - fix physical size calculation for 3rd-gen Bamboo

This calculation determines the physical dimensions of the tablet,
used later on in calculate_touch_res to obtain the touch sensor
resolution.

Instead of dividing the logical size by the resolution, the current
code performs a multiplication. This doesn't pose a problem for the
3rd-gen Bamboo since the resolution and scale factor happen to be
identical, but will produce an incorrect result for other cases.

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