net: bcmgenet: use the new fixed PHY helpers

of_phy_connect_fixed_link() is becoming obsolete, and also required
platform code to register the fixed PHYs at the specified addresses for
those to be usable. Get rid of it and use the new of_phy_is_fixed_link()
plus of_phy_register_fixed_link() helpers to transition over the new
scheme.

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