fix scan resolution support for Naga 2014

Fix #32: 
- the Naga 2014 actually support a maximum scan resolution of 8200 DPI
  (instead of 5600 DPI for earlier models)
- the format of the resolution command is also different

I tried to make the code generic so other models similar to the Naga 2014 can be added (e.g. the Naga Epic Chroma support 8200 DPI too according to its manual).

The 3rd commit cleanup the endianess handling to use big-endian only:
- that's what we need for setting the resolution with the Naga 2014 (2 halfwords; x and y)
- we use it for getting the firmware version
- it does not matter for the magic values
- for the other requests, the data is really independent bytes (e.g. 2 bytes for the resolution; x and y)