API hygiene. setADTR1107Mode flips ADTR1107 PA/LNA bias registers but
does NOT touch the per-channel ADAR1000 RX/TX enable bits. Production
always reaches it through setAllDevicesTXMode / setAllDevicesRXMode,
which emit both halves. Leaving setADTR1107Mode public after F-6.1
removed the other public mode-switch wrappers invited a future caller
to invoke it directly and end up in a mismatched bias-vs-enable state.
Move the declaration to the private section with a short comment
explaining why the wrappers are the only sanctioned entry point.
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