e1000: unmap ce4100_gbe_mdio_base_virt in e1000_remove

We are not unmapping ce4100_gbe_mdio_base_virt in exit path in case
we are running on a CE4100 adapter, fix that.

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>