uavcan: bug fix sensor bridge init failure
uavcan: bug fix sensor bridge init failure

Fixes case where a UAVCAN SensorBridge has callback channels available,
but NuttX / uORB does not have an additional driver / topic instance
available.

PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware