Fix VTF for QingKe-V2A/C

[Noxime]

Any previously set VTF channels got messed up by new writing.

The VTF code is broken in other ways too, but I'll wait for input on those:

1. V2A/C cores have 2 channels, the assertion is for < 4. Should new v2 feature be added?
2. V3A has different VTF system than V3B/C. Should those be broken out to v3a and v3bc features?