usb: gadget: Clear usb_endpoint_descriptor inside the struct usb_ep on disable

This fix a bug in f_serial, which expect the ep->desc to be NULL after
disabling an endpoint.

Cc: stable@vger.kernel.org
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/fsl_qe_udc.c b/drivers/usb/gadget/fsl_qe_udc.c
index b95697c..877a2c4 100644
--- a/drivers/usb/gadget/fsl_qe_udc.c
+++ b/drivers/usb/gadget/fsl_qe_udc.c
@@ -1638,6 +1638,7 @@
 	/* Nuke all pending requests (does flush) */
 	nuke(ep, -ESHUTDOWN);
 	ep->desc = NULL;
+	ep->ep.desc = NULL;
 	ep->stopped = 1;
 	ep->tx_req = NULL;
 	qe_ep_reset(udc, ep->epnum);