net: dsa/bcm_sf2: move speed/duplex forcing to mac_link_up()

Convert the bcm_sf2 to use the finalised speed and duplex in its
mac_link_up() call rather than the parameters in mac_config().

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>