usb: net: migrate USB Ethernet adapters to Kconfig

This migrates ASIX, ASIX88179, MCS7830, RTL8152 and SMSC95XX to Kconfig.
Update defconfigs.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx6slevk_spl_defconfig b/configs/mx6slevk_spl_defconfig
index fc16375..4d4f273 100644
--- a/configs/mx6slevk_spl_defconfig
+++ b/configs/mx6slevk_spl_defconfig
@@ -41,4 +41,5 @@
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_HOST_ETHER=y
+CONFIG_USB_ETHER_ASIX=y
 CONFIG_OF_LIBFDT=y