usb: dwc3: gadget: Allow restarting a transfer

It's possible that there's no new TRBs prepared when kicking a transfer.
This happens when we need to stop and restart a transfer such as in the
case of reinitiating a stream or retrying isoc transfer. For streams,
sometime host may reject a stream and the device may need to reinitiate
that stream by stopping and restarting a transfer. In this case, all the
TRBs may have already been prepared. Allow the function
__dwc3_gadget_kick_transfer() to go through even though there's no new
TRB.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/dwc3/gadget.c

@@ -1326,8 +1326,13 @@ static int __dwc3_gadget_kick_transfer(struct dwc3_ep *dep)
 	int				ret;
 	u32				cmd;
 
+	/*
+	 * Note that it's normal to have no new TRBs prepared (i.e. ret == 0).
+	 * This happens when we need to stop and restart a transfer such as in
+	 * the case of reinitiating a stream or retrying an isoc transfer.
+	 */
 	ret = dwc3_prepare_trbs(dep);
-	if (ret <= 0)
+	if (ret < 0)
 		return ret;
 
 	starting = !(dep->flags & DWC3_EP_TRANSFER_STARTED);