usb: gadget: function: u_ether: don't starve tx request queue

If we don't guarantee that we will always get an
interrupt at least when we're queueing our very last
request, we could fall into situation where we queue
every request with 'no_interrupt' set. This will
cause the link to get stuck.

The behavior above has been triggered with g_ether
and dwc3.

Cc: <stable@vger.kernel.org>
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_ether.c

@@ -590,8 +590,9 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 
 	/* throttle high/super speed IRQ rate back slightly */
 	if (gadget_is_dualspeed(dev->gadget))
-		req->no_interrupt = (dev->gadget->speed == USB_SPEED_HIGH ||
-				     dev->gadget->speed == USB_SPEED_SUPER)
+		req->no_interrupt = (((dev->gadget->speed == USB_SPEED_HIGH ||
+				       dev->gadget->speed == USB_SPEED_SUPER)) &&
+					!list_empty(&dev->tx_reqs))
 			? ((atomic_read(&dev->tx_qlen) % dev->qmult) != 0)
 			: 0;