net: phy: Fix the register offsets in Broadcom iProc mdio mux driver

Modify the register offsets in the Broadcom iProc mdio mux to start
from the top of the register address space.

Earlier, the base address pointed to the end of the block's register
space. The base address will now point to the start of the mdio's
address space. The offsets have been fixed to match this.

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>