Wh/km efficiency keep changing when aircraft is on the ground

[avsaase]

Current Behavior

When the aircraft is on the ground, and thus not flying, the Wh/km efficiency value on the OSD keep jumping around even though the indicated speed is zero. The mAh/km value correctly shows --- to indicate that this value cannot be calculated when speed is zero.

INAV_Whkm_bug.mp4

Steps to Reproduce

1. Put Wh/km efficiency on OSD
2. See that value is changing when aircraft is not flying

Expected behavior

When speed is zero (or maybe better a low value), display --- Wh/km.

Additional context

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• FC Board name and vendor: MATEK F765 Wing
• INAV version string: INAV/MATEKF765 2.6.0 Dec 29 2020 / 23:11:02 (d72b4649)

This is a custom firmware version to test some PR's but it is unlikely that these cause this bug.