px4io driver: Only try to upload if we have a non-zero failsafe throttle value
px4io firmware: Actually accept failsafe values, check their range. Add an error return code for commands that were rejected on reboot and force safety instead of failing silently with an OK value
Minor cleanups on handling, make wrong reg accesses non-fatal for the transfer
Merge pull request #964 from PX4/px4io_fixes

Px4io fixes