Fix properties for USBHost library

[cmaglie]

quoting Dirk67 on http://forum.arduino.cc/index.php?topic=330204.msg2281173#msg2281173:

so (as a small issue to mention) there is just some lack of documentation, I think
http://www.arduino.cc/en/Reference/USBHost -> is listed as "Due only" e.g.
and no word of ADK and so on ...

and even in the text files .properties and comments here
https://github.com/arduino/ArduinoCore-samd/tree/master/libraries/USBHost
is written "For Arduino DUE only".