net: phy: adjust fixed_phy_register() return value

Adjust fixed_phy_register() to return struct phy_device *, so that
it becomes easy to use fixed PHYs without device tree support:

  phydev = fixed_phy_register(PHY_POLL, &fixed_phy_status, NULL);
  fixed_phy_set_link_update(phydev, fixed_phy_link_update);
  phy_connect_direct(netdev, phydev, handler_fn, phy_interface);

This change is a prerequisite for modifying bcmgenet driver to work
without a device tree on Broadcom's MIPS-based 7xxx platforms.

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>