decklink: Make sure to return a value from all code paths

False warning from MSVC, or it does not understand that
g_assert_not_reached() does not return.

...\gst-plugins-bad-1.0-1.17.0.1\sys\decklink\gstdecklink.cpp(1647) : warning C4715: 'gst_decklink_configure_duplex_mode': not all control paths return a value

sys/decklink/gstdecklink.cpp

@@ -1644,6 +1644,9 @@ gst_decklink_configure_duplex_mode (Device * device, BMDDuplexMode duplex_mode)
       return DUPLEX_MODE_SET_FAILURE;
     }
   }
+
+  g_assert_not_reached ();
+  return DUPLEX_MODE_SET_FAILURE;
 }
 
 DuplexModeSetOperationResult