[media] em28xx: ignore isoc DVB USB endpoints with wMaxPacketSize = 0 bytes for all alt settings

Some devices without DVB support (such as the "Terratec Grabby" and
"Easycap DC-60") provide isochronous DVB USB endpoints with wMaxPacketSize set
to 0 bytes for all alt settings.
Ignore these endpoints and avoid registering a DVB device node and loading the
DVB driver extension.
Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Tested-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/em28xx/em28xx-cards.c

@@ -3357,14 +3357,15 @@ static int em28xx_usb_probe(struct usb_interface *interface,
 						dev->analog_ep_bulk =
 							    e->bEndpointAddress;
 					} else {
-						has_dvb = true;
 						if (usb_endpoint_xfer_isoc(e)) {
 							dev->dvb_ep_isoc = e->bEndpointAddress;
 							if (size > dev->dvb_max_pkt_size_isoc) {
+								has_dvb = true; /* see NOTE (~) */
 								dev->dvb_max_pkt_size_isoc = size;
 								dev->dvb_alt_isoc = i;
 							}
 						} else {
+							has_dvb = true;
 							dev->dvb_ep_bulk = e->bEndpointAddress;
 						}
 					}
@@ -3391,6 +3392,12 @@ static int em28xx_usb_probe(struct usb_interface *interface,
 			 * so far. But there might be devices for which this
 			 * logic is not sufficient...
 			 */
+			/*
+			 * NOTE (~): some manufacturers (e.g. Terratec) disable
+			 * endpoints by setting wMaxPacketSize to 0 bytes for
+			 * all alt settings. So far, we've seen this for
+			 * DVB isoc endpoints only.
+			 */
 		}
 	}