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/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index a696bde..a1a04d0 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -440,6 +440,7 @@
 
 	dep->stream_capable = false;
 	dep->desc = NULL;
+	dep->endpoint.desc = NULL;
 	dep->comp_desc = NULL;
 	dep->type = 0;
 	dep->flags = 0;