igb: fix init on 82575 with MNG enabled

This patch resolves an issue seen on 82575 adapters with managability
pass-thru enabled, which could cause the system to panic.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>