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>