tablet: disable the forced proximity out for the Dell Canvas pen

This pen has random timeouts, often when a button is pressed. This causes a
forced proximity out (and the button release) and makes the whole device a
tad unusable.

Nothing we can detect by heuristics since it looks like other devices that
don't send proximity out events. And the timeout can be quite high, the
recording in #304 has over 800ms for one sequence.

Fixes #304

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>