The Aiptek 8000U has a pressure offset above our default (%5) but no
meaningful way of detecting that. It doesn't provide distance or BTN_TOOL_PEN
either, so our heuristics can't hook onto anything. BTN_TOUCH is set by this
tablet but not at consistent pressure thresholds.

Work around this by shipping a quirk that ups it to 70. Aiptek
re-uses USB IDs because of course they do, so this applies to more than one
device. Let's see what breaks.

Fixes #462

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