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/nitrogen6dl_defconfig b/configs/nitrogen6dl_defconfig
index 61e5ea4..a20580e 100644
--- a/configs/nitrogen6dl_defconfig
+++ b/configs/nitrogen6dl_defconfig
@@ -51,5 +51,6 @@
 CONFIG_G_DNL_MANUFACTURER="Boundary"
 CONFIG_G_DNL_VENDOR_NUM=0x0525
 CONFIG_G_DNL_PRODUCT_NUM=0xa4a5
+CONFIG_USB_ETHER=y
 # CONFIG_VIDEO_SW_CURSOR is not set
 CONFIG_OF_LIBFDT=y