ag71xx: close a race between the phy state machine and link state

ag71xx: close a race between the phy state machine and link state

A fast stop/start cycle could leave the ag71xx interrupts and tx engine
disabled when using a phy driver with a fixed link and the start/stop
happens between two phy state machine polls.

Prevent this by always forcing the link down on stop regardless of phy
state and having a phy connected.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28380 3c298f89-4303-0410-b956-a3cf2f4a3e73