Improve update performance and clean 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.
- Tested on V1 and V2 platforms