Fixes case where a UAVCAN SensorBridge has callback channels available,
but NuttX / uORB does not have an additional driver / topic instance
available.
Supposedly multiple sensor callbacks were supported; in reality, this
was not the case, as the mag SensorBridge in particular can only
calibrate one compass, leading to a race condition on which compass
appears first on the bus to get published and calibrated (with no
warning to the user that the 'wrong' compass is being used).
For sensors with existing generic driver classes (baro and mag) the
sensor bridges use these classes for the driver registration, and uORB
publication, and calibration interface (ioctl) handling.
UavcanGnssBridge did not support more than 1 GNSS callback/publisher.
This has now been fixed; it works the same as the baro, mag, and flow
sensor bridges. The EKF2 still doesn't support more than 2 GPS
publishers, however.
JacobCrabill