AP_HAL_PX4: fix resetting TX buffer

We can't give the TX buffer 16 bytes more since next time begin() is
called it will compare the buffer size to the value the caller is trying
to set.  In this case we would free and alloc the buffer again each time
begin was called.

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -90,7 +90,7 @@ void PX4UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
         while (_in_timer) {
             hal.scheduler->delay(1);
         }
-        _writebuf.set_size(txS+16);
+        _writebuf.set_size(txS);
     }
 
 	if (_fd == -1) {