usb: gadget: Move USBNET_HOST_ADDR to Kconfig

While the USB Ethernet device address is already defined in Kconfig, the
host address isn't. Convert it.

Reviewed-by: Łukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>