netifd: bring wifi down before shutting down

works around wifiX references not being freed on network restart.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36883 3c298f89-4303-0410-b956-a3cf2f4a3e73