USB: fsl_qe_udc: Fix stalled TX requests bug

While disabling an endpoint the driver nuking any pending requests,
thus completing them with -ESHUTDOWN status. But the driver doesn't
clear the tx_req, which means that a next TX request (after
ep_enable), might get stalled, since the driver won't queue the new
reqests.

This patch fixes a bug I'm observing with ethernet gadget while
playing with ifconfig usb0 up/down (the up/down sequence disables
and enables `in' and `out' endpoints).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/fsl_qe_udc.c

@@ -1622,6 +1622,7 @@ static int qe_ep_disable(struct usb_ep *_ep)
 	nuke(ep, -ESHUTDOWN);
 	ep->desc = NULL;
 	ep->stopped = 1;
+	ep->tx_req = NULL;
 	qe_ep_reset(udc, ep->epnum);
 	spin_unlock_irqrestore(&udc->lock, flags);