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/thuban_defconfig b/configs/thuban_defconfig
index c30e924..eb9124c 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -70,3 +70,4 @@
 CONFIG_G_DNL_MANUFACTURER="Siemens AG"
 CONFIG_G_DNL_VENDOR_NUM=0x0908
 CONFIG_G_DNL_PRODUCT_NUM=0x02d2
+CONFIG_USB_ETHER=y