usb: gadget: usb_ether: Move settings to common

The usb_ether gadget duplicates the USB settings for the manufacturer,
product ID and vendor ID.

Make sure we use the common option so that we can expect a single VID/PID
couple for a single device.

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