ADSB_Avoid Mode puts aircraft in hover

#### Issue details
ADSB_Avoid Mode puts aircraft into hover, regardless of breach of other ADSB aircraft occurring or even avoidance being enabled (avd_enable: 0). Hover allows no thrust, pitch, or roll input from operator but does allow yaw input. This bug appeared to occur after setting the avoidance action for ADSB to hover (avd_f_action: 6). Rebooting power, Reflashing copter firmware to pixhawk, and changing back to previously working avd parameters were all unsuccessful in returning ADSB_avoidance to a previously working state.

Prior to setting this parameter, putting the aircraft into ADSB_Avoid mode appeared to assign the aircraft to the previous flight mode with avoidance only occurring when the ADSB fail-safe parameters were met (AVD_ENABLE, AVD_F_DIS_XY, AVD_F_DIS_Z, AVD_F_TIME). Avoidance was occurring and appeared successful in Stabilize, Auto, Guided, Loiter, Alt Hold. Avd_f_action used for these modes was RTL. Other actions tested with ADSB avoidance (avd_f_action) were Report, Climb or Descend, and hover. The bug appeared when testing hover as the avd_f_action and has not been resolved.

Hardware
Avoiding aircraft
F450 DJI airframe
Pixhawk 2.1
uAvionix Ping 2020 (integrated with autopilot,reporting, and previously avoiding ADSB-out equipped aircraft)

Intruding Aircraft
DJI S800 Frame
APM 2.6 Autopilot
uAvionix Ping 2020 (transmitting only: not integrated with autopilot)

#### Version
Copter V3.5.5

Copter
[ ] All
[ ] AntennaTracker
[X] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Quad_X

#### Hardware type
Pixhawk 2.1

#### Logs
Note: Intruding aircraft does not appear on t-log but all RTLs occurred when ADSB_FS_ACTION set as RTL
[Successful ADSB tests.zip](https://github.com/ArduPilot/MissionPlanner/files/1885805/Successful.ADSB.tests.zip)

No source files, tests, or entry points are named. Start by reviewing the ADSB_Avoid and avd_enable, avd_f_action, and ADSB fail-safe parameter behavior described in the report, then inspect the attached successful ADSB tests archive. Done means identifying why the vehicle enters hover without an applicable breach or enabled avoidance and confirming the behavior against the reported hardware and Copter 3.5.5 setup.

Written by the indexing model from the issue text.