sensortype should enumerate to ULTRASOUND in ping1d_mavlink_driver.py

[FletcherFT]

Line 103 of tools/ping1d_mavlink_driver.py:

sensor_type = 2

This enumerates to INFRARED according to Mavlink distance sensor specification and causes errors when using MAVROS to publish distance_sensor messages.

I think the enumerator should be 1 for ULTRASOUND enumeration. Setting sensor_type = 1 allows MAVROS to convert the distance_sensor message correctly.