igb: improve handling of disconnected adapters

Clean up array_rd32 so that it uses igb_rd32 the same as rd32, per the
suggestion of Alexander Duyck, and use io_addr in more places, so that
we don't have the need to call E1000_REMOVED (which simply looks for a
null hw_addr) nearly as much.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>