usbdev: extend the usb req len to size_t

Some USB controllers can receive or send multiple data packets then
generate one interrupt. This mechanism can reduce the number of data
copies. Extend req buf to accommodate this.

Signed-off-by: yangsong8 <yangsong8@xiaomi.com>

drivers/usbdev/usbdev_req.c

@@ -68,7 +68,7 @@
  ****************************************************************************/
 
 FAR struct usbdev_req_s *usbdev_allocreq(FAR struct usbdev_ep_s *ep,
-                                         uint16_t len)
+                                         size_t len)
 {
   FAR struct usbdev_req_s *req;