HAL_PX4: added some UART debugging

this may help to track down USB issues

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -110,6 +110,10 @@ void PX4UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
 	}
 
     if (_writebuf_size != 0 && _readbuf_size != 0 && _fd != -1) {
+        if (!_initialised) {
+            ::printf("initialised %s OK %u %u\n", _devpath, 
+                     (unsigned)_writebuf_size, (unsigned)_readbuf_size);
+        }
         _initialised = true;
     }
 }