usb: gadget: Move USBNET_HOST_ADDR to Kconfig

While the USB Ethernet device address is already defined in Kconfig, the
host address isn't. Convert it.

Reviewed-by: Ɓukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/configs/rastaban_defconfig b/configs/rastaban_defconfig
index 5acfe22..940c414 100644
--- a/configs/rastaban_defconfig
+++ b/configs/rastaban_defconfig
@@ -70,3 +70,4 @@
 CONFIG_G_DNL_MANUFACTURER="Siemens AG"
 CONFIG_G_DNL_VENDOR_NUM=0x0908
 CONFIG_G_DNL_PRODUCT_NUM=0x02d2
+CONFIG_USBNET_HOST_ADDR="de:ad:be:af:00:00"