igb: cleanup usage of virtualization registers

The igb driver was incorrectly attempting to write to registers that do not
exist on 82580 hardware.  This wasn't causing any issues that I can tell,
but it is not recommended behavior either so I have changed this so we are
determining which registers to write to based on mac type.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>