fixed a warning

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2486 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/drivers/usb/usbdevice/class/cdc_vcom.c

@@ -226,7 +226,6 @@ static rt_err_t _cdc_get_line_coding(udevice_t device, ureq_t setup)
 {
     struct ucdc_line_coding data;
     rt_uint16_t size;
-    rt_err_t ret;
     
     RT_ASSERT(device != RT_NULL);
     RT_ASSERT(setup != RT_NULL);