Param1 and param2 reversed

Param1 should be for WP radius and param2 for the time inside the radius

This is according to:
https://github.com/PX4/Firmware/blob/master/mavlink/include/mavlink/v1.0/common/mavlink_msg_mission_item.h#L7

It is also reversed in the PX4 Firmware:
https://github.com/PX4/Firmware/blob/master/src/modules/mavlink/waypoints.c#L325

So please hold back with the merge until it is fixed there as well.