net: dsa/bcm_sf2: fix incorrect usage of state->link

state->link has never been valid in mac_config() implementations -
while it may be correct in some calls, it is not true that it can be
relied upon.

Fix bcm_sf2 to use the correct method of handling forced link status.

Reviewed-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>