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/sansa_fuze_plus_defconfig b/configs/sansa_fuze_plus_defconfig
index f6ce09c..c6d50cc 100644
--- a/configs/sansa_fuze_plus_defconfig
+++ b/configs/sansa_fuze_plus_defconfig
@@ -32,4 +32,5 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_GADGET=y
 CONFIG_CI_UDC=y
+CONFIG_USB_ETHER=y
 CONFIG_OF_LIBFDT=y