tests: drivers: build_all: ethernet: Enable full-duplex of enc28j60

Following the change that replaces ETH_ENC28J60_0_FULL_DUPLEX
by devicetree property.
Enable enc28j60's `full-duplex` explicitly in the overlay file.

Signed-off-by: TOKITA Hiroshi <[email protected]>

Author: soburi

tests/drivers/build_all/ethernet/app.overlay

@@ -39,6 +39,7 @@
 			test_spi_enc28j60: enc28j60@0 {
 				compatible = "microchip,enc28j60";
 				reg = <0x0>;
+				full-duplex;
 				spi-max-frequency = <0>;
 				int-gpios = <&test_gpio 0 0>;
 				local-mac-address = [00 00 00 00 00 00];