workaround RADIO_CONTROL_NB_CHANNEL for dual mcu

radio_control driver and radio.h (generated) sometimes define different RADIO_CONTROL_NB_CHANNELS.

e.g. SBUS -> 16 channels, but radio file could be less.

Suggestion: RADIO_CONTROL_NB_CHANNELS should always come from the generated/radio.h and the driver locally can have as many channels as they like