e1000: cleanup CE4100 MDIO registers access

A global variable is currently used to hold the virtual address of the
CE4100 MDIO base register address. Store the address in the e1000_hw
structure and update macros accordingly.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>