phy: mdiobus_register(): initialize all phy_map entries

make sure phy_map entries whose PHY address is masked are initialized
to NULL, given that other code (such as mdiobus_unregister for
instance) assumes that non-NULL phy_map entries are allocated
phy_devices

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>