usb: net: migrate CONFIG_USB_HOST_ETHER to Kconfig

CONFIG_USB_HOST_ETHER is the framework that the drivers are dependent on
USB_HOST_ETHER. Use this as a menu and move the existing LAN75XX and
LAN78XX options under new menu. Finally update the defconfigs that need
CONFIG_USB_HOST_ETHER.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/smartweb_defconfig b/configs/smartweb_defconfig
index 8703316..8e16d9f 100644
--- a/configs/smartweb_defconfig
+++ b/configs/smartweb_defconfig
@@ -50,4 +50,5 @@
 CONFIG_G_DNL_MANUFACTURER="Siemens AG"
 CONFIG_G_DNL_VENDOR_NUM=0x0908
 CONFIG_G_DNL_PRODUCT_NUM=0x02d2
+CONFIG_USB_HOST_ETHER=y
 # CONFIG_EFI_LOADER is not set