of/mdio: fix fixed link bus name

Since 9e6c643b (phy/fixed: use an unique MDIO bus name) the name of the fixed
PHY bus is "fixed-0". Teach of_phy_connect_fixed_link() the new name.

Tested on a P1020RDB PowerPC system.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>