ar71xx: ag71xx: fix ag71xx_probe error path

Signed-off-by: Catalin Patulea <cat@vv.carleton.ca>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38716 3c298f89-4303-0410-b956-a3cf2f4a3e73