UAVCAN made warning-free

There's no significant changes to the PX4 codebase.

Probably PX4 could benefit from https://github.com/pavel-kirienko/uavcan/blob/master/libuavcan/include/uavcan/util/comparison.hpp#L30 or https://github.com/pavel-kirienko/uavcan/blob/master/libuavcan/include/uavcan/util/comparison.hpp#L157.