[radio_control] cleanup channel defines

- PPM: remove PPM_NB_CHANNEL define an always decode RADIO_CTL_NB (number of channels in radio.xml)
- PPM: the actually used/needed channels RADIO_CONTROL_NB_CHANNEL default to RADIO_CTL_NB but can be overwritten by user
  e.g. to use less channels for intermcu
- SBUS: only provide a default for RADIO_CONTROL_NB_CHANNEL

As far as I can discern this should finally make it possible to override the actually used RADIO_CONTROL_NB_CHANNEL from the airframe file (e.g. to transfer less via intermcu) while still using the proper number of channels for parsing.
So with this it looks like the separate RADIO_CONTROL_USED_NB_CHANNELS as proposed in #973 is not needed.
But maybe we still want to check that RADIO_CONTROL_NB_CHANNEL <= implementation specific max number of channels...

Untested on real hardware...