Sik Radio Flight Controller Passthrough Not Working

#### Issue details
Trying to update a radio connected to a flight controller did not work no matter what settings I tried. @tridge advised that this was once possible, but also could not connect.

Tried "Load Settings" straight away after plugging USB into cube, and also tried connecting to cube and then loading settings. Tried all different baud rates, etc. and nothing worked. It actually crashes MP when trying to load settings after connecting via USB.

#### Version
1.3.57 beta - downloaded 8/1/18

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ x ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Standard Plane

#### Hardware type
Cube Black. 3DR SiK Radios

#### Logs
N/A

Start by reproducing the failure in Mission Planner 1.3.57 beta with a Cube Black and 3DR SiK radios, using the USB connection and the “Load Settings” action described here. The issue names no source files or tests; done means radio passthrough and settings loading work without Mission Planner crashing.

Written by the indexing model from the issue text.