net: phy: provide stub for fixed_phy_set_link_update

In preparation for updating the DSA code and avoid using ifdefs there,
provide an empty stub for fixed_phy_set_link_update when
CONFIG_FIXED_PHY is not selected.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>