usb: dwc3: drd: Improve dwc3_get_extcon() style

The previous change ("usb: dwc3: drd: Avoid error when extcon is
missing") changed the code flow in dwc3_get_extcon() function, leading
to unnecessary if-branch. This patch does housekeeping by reworking the
code for obtaining an extcon device from the "port" node.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20201214110741.8512-3-semen.protsenko@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>