device_impl: bugfix for missing arm/takeoff result
device_impl: bugfix for missing arm/takeoff result

This fixes a newly introduced bug where the arm and takeoff async
commands didn't correctly call the callback. The problem was that the
_command_result_callback got overwritten if in the callback itself a new
command was sent.

dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore