net: phy: Disable external master access in bcm mdio mux driver

Configure the scan control register in the Broadcom iProc
mdio mux driver to disable access to external master.

In some SoC's, the scan control register defaults to an incorrect
value.

Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>