bsp: same70_xplained: fix unused variable warning

Signed-off-by: Rafael Silva <perigoso@riseup.net>

hw/bsp/same70_xplained/same70_xplained.c

@@ -52,6 +52,7 @@ static volatile bool uart_busy = false;
 
 static void tx_cb_EDBG_COM(const struct usart_async_descriptor *const io_descr)
 {
+  (void) io_descr;
   uart_busy = false;
 }