Fix OSD speed unit preview

[McGiverGim]

The GPS Speed OSD element preview is wrong, depending on the units.
It is showing km/h for British, and mp/h for Imperial and Metric.

This PR changes this, using km/h for Metric, and mp/h for Imperial and British, like in the firmware part:
https://github.com/betaflight/betaflight/blob/025ee87a7aca068e3659fd066b8a9afbed123361/src/main/osd/osd_elements.c#L492-L505

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.2% Duplication

[haslinghuis]

Have implemented #2146 on user request. The user was from GB and insisted on using km for GPS speed. But it looks like British people are using one or the other depending on birthday?

[McGiverGim]

The actual code was wrong indeed. It was showing bad the units for Metric.
Now we can discuss if British/Imperial must shown one or another and I can modify the PR, but I have applied the same than firmware if I've looked at the correct code. If something is wrong then it must be changed on firmware first.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs within a week.

[haslinghuis]

Compared with firmware and you are indeed correct.
My bad it was one of my first code PR's 😃

[blckmn]

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