Improve update performance and clean up compiler warnings in px4io driver

- Fix compiler warnings in px4io_serial.cpp
- Fix compiler warnings in px4io_uploader.cpp
- Rename confusing overloaded send method with nearly identical
parameters in px4io_uploader.cpp
- Improve update performance by using maximum size programming buffer
since we are no longer limited by stack size.
Revert to original loop

Original loop was correct, and slightly more efficient. Retain
initialization of ret to handle the case where passed in count is 0.
Merge pull request #1232 from jean-m-cyr/master

Improve update performance and clean compiler warnings in px4io driver