Fixed PHY: Add fixed_phy_change_carrier()

Drivers can use this as .ndo_change_carrier() to change carrier
via /sys/class/net/ethX/carrier.

Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>