usb: gadget: Convert USB_ETHER to Kconfig

The USB Ethernet gadget option has not yet been moved to Kconfig, let's
deal with that.

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/warp7_defconfig b/configs/warp7_defconfig
index f430f30..8a3717f 100644
--- a/configs/warp7_defconfig
+++ b/configs/warp7_defconfig
@@ -38,5 +38,6 @@
 CONFIG_G_DNL_MANUFACTURER="FSL"
 CONFIG_G_DNL_VENDOR_NUM=0x0525
 CONFIG_G_DNL_PRODUCT_NUM=0xa4a5
+CONFIG_USB_ETHER=y
 CONFIG_USBNET_HOST_ADDR="de:ad:be:af:00:00"
 CONFIG_OF_LIBFDT=y