port-serial: fix warnings with -Wimplicit-fallthrough

mm-port-serial.c: In function ‘port_serial_process_command’:
  mm-port-serial.c:605:16: error: this statement may fall through [-Werror=implicit-fallthrough=]
    605 |             if (written > 0) {
        |                ^
  mm-port-serial.c:611:9: note: here
    611 |         case G_IO_STATUS_AGAIN:
        |         ^~~~

src/mm-port-serial.c

@@ -606,7 +606,8 @@ port_serial_process_command (MMPortSerial *self,
                 ctx->idx += written;
                 break;
             }
-            /* If written == 0, treat as EAGAIN, so fall down */
+            /* If written == 0 treat as EAGAIN */
+            /* Fall through */
 
         case G_IO_STATUS_AGAIN:
             /* We're in a non-blocking channel and therefore we're up to receive