phylink: ensure we take the link down when phylink_stop() is called

Ensure that we tell the MAC to take the link down when phylink_stop()
is called, and that this completes prior to phylink_stop() returns.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>