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/pxm2_defconfig b/configs/pxm2_defconfig
index b1da71d..b6624f0 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -74,6 +74,7 @@
 CONFIG_G_DNL_MANUFACTURER="Siemens AG"
 CONFIG_G_DNL_VENDOR_NUM=0x0908
 CONFIG_G_DNL_PRODUCT_NUM=0x02d2
+CONFIG_USB_ETHER=y
 CONFIG_USBNET_HOST_ADDR="de:ad:be:af:00:00"
 # CONFIG_VIDEO_SW_CURSOR is not set
 CONFIG_SYS_CONSOLE_BG_COL=0xff