ar71xx: implement switch fix for WD My Net N750

ar71xx: implement switch fix for WD My Net N750

The bootloader on the WD My Net N750 disables the ports on it's internal
AR8327N switch by powering them down. The stock firmware then brings the
ports back up again by starting the auto negotiation process on each
port.

This fix implements just that.

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>
Patchwork: http://patchwork.openwrt.org/patch/4615/
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39128 3c298f89-4303-0410-b956-a3cf2f4a3e73