USB_RF device: fixed message format

* USART Message format   [0: TYPE][1..16: MESSAGE           ] (17 bytes)
* USART Quality request  [0: '?' ][1..16: 0                 ] (17 bytes)
* USART Quality response [0: '+' ][1: RES][2: RETR][3..16: 0] (17 bytes)