usb: gadget: Document USBNET_DEVADDR

Add an help about the USBNET_DEVADDR Kconfig option to make it clearer what
it's about.

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