VW: allow adjusting set speed when disabled

[RJWadley]

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

some cars have +/- buttons that are separate. realistically for openpilot this just means VW, since most other affected cars makes use flexray, but it's not inherently a VW thing. KIA EV9 has independent buttons for example, but it treats them as SET so they're probably 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 +/- when disabled
• if we don't have an ACC setpoint, init one
• tweak adjustment handling so it only ignores button presses that cross a state change

VW MQB	KIA EV6

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

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