usb: dwc3: drd: Avoid error when extcon is missing

If "port" node is missing in PHY controller node, dwc3_get_extcon()
isn't able to find extcon devices. This is perfectly fine in case when
"usb-role-switch" or OTG is used, but next misleading error message is
printed in that case, from of_graph_get_remote_node():

    OF: graph: no port node found in /phy@1234abcd

Avoid printing that message by checking if the port node exists in PHY
node before calling of_graph_get_remote_node(). While at it, add the
comment from mentioned code block, explaining how checking the port
availability helps to avoid the misleading error.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20201214110741.8512-2-semen.protsenko@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/drd.c

@@ -462,8 +462,18 @@ static struct extcon_dev *dwc3_get_extcon(struct dwc3 *dwc)
 		return edev;
 	}
 
+	/*
+	 * Try to get an extcon device from the USB PHY controller's "port"
+	 * node. Check if it has the "port" node first, to avoid printing the
+	 * error message from underlying code, as it's a valid case: extcon
+	 * device (and "port" node) may be missing in case of "usb-role-switch"
+	 * or OTG mode.
+	 */
 	np_phy = of_parse_phandle(dev->of_node, "phys", 0);
-	np_conn = of_graph_get_remote_node(np_phy, -1, -1);
+	if (of_graph_is_present(np_phy))
+		np_conn = of_graph_get_remote_node(np_phy, -1, -1);
+	else
+		np_conn = NULL;
 
 	if (np_conn)
 		edev = extcon_find_edev_by_node(np_conn);