Rename the downstream ag71xx driver to ag71xx-legacy.
It allows both upstream and downstream drivers to be compiled into modules.
Signed-off-by: Oskari Lemmela <oskari@lemmela.net>
Link: https://github.com/openwrt/openwrt/pull/15926
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>