AP_Airspeed: allow manual selection of PX4 diff pressure sensor scale.

User can now set the pressure scaling for the PX4 airspeed sensor using
ARSPD_SENS_SCALE parameter. This allows for the code to work on all of
the MEAS MS4525DO and Honeywell HSC/SSC differential pressure sensors,
rather than just the +/-1 PSI model. Unfortunately it can't be done
automatically because all the sensors have the same I2C address and the
same register layout.

For all boards other than the PX4/Pixhawk, this setting is ignored. For the
PX4/Pixhawk, the default scale remains the same (ie correct for the +/- 1PSI
MS4525DO). If the user doesn't manually set anything, they should not
notice any difference at all.

There's an associated update for the PX4Firmware that I've submitted another
pull request for. That update is necessary before this one will work correctly.