net: phy: Initialize mdio clock at probe function

USB PHYs need the MDIO clock divisor enabled earlier to work.
Initialize mdio clock divisor in probe function. The ext bus
bit available in the same register will be used by mdio mux
to enable external mdio.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <yendapally.reddy@broadcom.com>
Fixes: ddc24ae1 ("net: phy: Broadcom iProc MDIO bus driver")
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>