usb: gadget: usb_ether: Move the interfaces to Kconfig

We need to select an interface for the usb_ether gadget, and they haven't
been converted to Kconfig yet. Add a choice to make sure we have an option
selected, and convert all the users.

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