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/sama5d2_ptc_spiflash_defconfig b/configs/sama5d2_ptc_spiflash_defconfig
index ad62f47..3cb9b36 100644
--- a/configs/sama5d2_ptc_spiflash_defconfig
+++ b/configs/sama5d2_ptc_spiflash_defconfig
@@ -31,3 +31,4 @@
 CONFIG_USB_STORAGE=y
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_ATMEL_USBA=y
+CONFIG_USB_ETHER=y