net: axienet: use resolved link config in mac_link_up()

Convert the Xilinx AXI ethernet driver to use the finalised link
parameters in mac_link_up() rather than the parameters in mac_config().

Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>