usb: gadget: dummy_hcd: emulate sending zlp in packet logic

currently, when a zlp flag is set and an urb/usb_request
buffer is filled without a short packet, transfer() leaves
its status at -EINPROGRESS and does not rescan for short
packet.

In a scenario where ep.maxpacket bytes are copied,
URB_ZERO_PACKET is set, urb buffer is filled and usb_request
buffer is not, transfer() returns with an urb with
-EINPROGRESS status, which dummy_hcd treats as incomplete
transfer.

Check for zlp and rescan appropriately.
Signed-off-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/udc/dummy_hcd.c

@@ -1429,16 +1429,25 @@ static int transfer(struct dummy_hcd *dum_hcd, struct urb *urb,
 					req->req.status = 0;
 			}
 
-		/* many requests terminate without a short packet */
+		/*
+		 * many requests terminate without a short packet.
+		 * send a zlp if demanded by flags.
+		 */
 		} else {
-			if (req->req.length == req->req.actual
-					&& !req->req.zero)
+			if (req->req.length == req->req.actual) {
+				if (req->req.zero && to_host)
+					rescan = 1;
+				else
 					req->req.status = 0;
-			if (urb->transfer_buffer_length == urb->actual_length
-					&& !(urb->transfer_flags
-						& URB_ZERO_PACKET))
+			}
+			if (urb->transfer_buffer_length == urb->actual_length) {
+				if (urb->transfer_flags & URB_ZERO_PACKET &&
+				    !to_host)
+					rescan = 1;
+				else
 					*status = 0;
 			}
+		}
 
 		/* device side completion --> continuable */
 		if (req->req.status != -EINPROGRESS) {