rt_kprintf: force the open_flag to be RT_DEVICE_FLAG_STREAM

Commit a6251a43 changed the serial device to check open_flag
instead of flag. So we should change the rt_kprintf counterpart as well.

src/kservice.c

@@ -1114,11 +1114,11 @@ void rt_kprintf(const char *fmt, ...)
     }
     else
     {
-        rt_uint16_t old_flag = _console_device->flag;
+        rt_uint16_t old_flag = _console_device->open_flag;
 
-        _console_device->flag |= RT_DEVICE_FLAG_STREAM;
+        _console_device->open_flag |= RT_DEVICE_FLAG_STREAM;
         rt_device_write(_console_device, 0, rt_log_buf, length);
-        _console_device->flag = old_flag;
+        _console_device->open_flag = old_flag;
     }
 #else
     rt_hw_console_output(rt_log_buf);