VW: allow adjusting set speed when disabled

[RJWadley]

goal: more closely align with stock ACC behavior on VW cars with unique buttons

some cars have +/- buttons that are separate. realistically for openpilot this just means VW, since most other affected cars use flexray, but it's not inherently a VW thing. KIA EV9 has independent buttons for example, but it still treats them as SET+ and SET- so they're not affected here. VW's stock system allows adjustment of the set point while disabled. In most cars, the buttons are shared with RES/SET and this is inherently not possible.

tweaks:

• handle cruise +/- presses when disabled
• if we attempt to adjust speed but don't have an ACC setpoint, init one
• tweak speed adjustment handling to ignore any button press that could enable openpilot

VW MQB	KIA EV6

Sample route showing adjustment while disabled:
a3d353f2254caee4/00000242--cb0f881531/0

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