Eagle Tree Pilot Tube Sensor: Fix calibration and airspeed computation.

The sensor was being miscalibrated because the division was wrong. This
fixes the issue, as well as cleans up the defines.

It also changes the airspeed calculation to occur only when the pressure
is positive.
Merge pull request #1002 from kubark42/fix-eagle-tree-calibration

Eagle Tree Pilot Tube Sensor: Fix calibration and airspeed computation.