DShot throttle unavailable

#### Issue details
I fly planes and need to monitor throttle for plane health. CH3_Percent does not work for DShot ESC's

#### Version
1.3.56.4

Plane 3.9 beta 1

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

#### Airframe type
Twin engine electric V-Tail

#### Hardware type
Cube, RFD900, BLHeli_32 ESC's

#### Logs

Start by reproducing the issue on Plane 3.9 beta 1 with the listed Cube, RFD900, and BLHeli_32 ESC setup, then trace where CH3_Percent is obtained or displayed for DShot ESCs. There are no files or tests named in the report; done means establishing why the throttle value is unavailable and verifying the corrected value on a comparable setup.

Written by the indexing model from the issue text.