Tests on all Linux machines available led to errors such as:

  Erasing the memory ...
  erasing: 66% complete
  Error erasing: -84>> Abort mission! error has occured ...
  Please unplug the device and restart the program.

Whereas Windows can recover from this condition with

  Eep! Connection to device lost during erase! Not to worry
  This happens on some computers - reconnecting...

the Linux version could not correctly interpret the -84 error code returned by the USBDEVFS_CONTROL call.

This modification fixes this, re-interpreting the -84 code to -2 and allowing the program to reconnect and succesfully upload the firmware.
Merge pull request #20 from psychogenic/master

Fix for linux erase/write issue.