usb: gadget: usb_ether: Move the interfaces to Kconfig

We need to select an interface for the usb_ether gadget, and they haven't
been converted to Kconfig yet. Add a choice to make sure we have an option
selected, and convert all the users.

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/nitrogen6q2g_defconfig b/configs/nitrogen6q2g_defconfig
index 940c74a..f9160c2 100644
--- a/configs/nitrogen6q2g_defconfig
+++ b/configs/nitrogen6q2g_defconfig
@@ -53,5 +53,6 @@
 CONFIG_G_DNL_VENDOR_NUM=0x0525
 CONFIG_G_DNL_PRODUCT_NUM=0xa4a5
 CONFIG_USB_ETHER=y
+CONFIG_USB_ETH_CDC=y
 # CONFIG_VIDEO_SW_CURSOR is not set
 CONFIG_OF_LIBFDT=y