fix cluster displayed set speed skipping certain mph values

[gregjhogan]

When incrementing set speed, convert to display units, increment in display units, then convert back to original units.

No clusters display set speed units with decimal places, and this ensures when you convert the stored value to any units and round to the nearest integer you always get the desired set speed value.

• Hyundai simply requires we send the correct value in display units, and previously the higher the set speed the further off the conversion back to mph gets until it rounded down to the incorrect value. (e.g. set speeds above 85 mph)
• Honda requires the kph value we send divided by 1.6 to round to the mph value, and previously all the conversions going on introduced error (e.g. it would never show a set speed of 65 mph)
• Volkswagen also has issues based on this comment?
  

  openpilot/selfdrive/car/volkswagen/carcontroller.py

  Line 102 in 0907b30

  # FIXME: follow the recent displayed-speed updates, also use mph_kmh toggle to fix display rounding problem?

[gregjhogan]

Tested on 2016 civic, 2018 accord, and 2022 ev6 with hda2 using openpilot longitudinal and the set speed seems to always be correct now.

[sshane]

@jyoung8607 can you try this?

[jyoung8607]

@jyoung8607 can you try this?

I will be test-driving some harness samples in the next couple days, I'll validate this at the same time.

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