igb: make dev_spec a union and remove dynamic allocation

This patch makes dev_spec a union and simplifies it so that it does not
require dynamic allocation and freeing in the driver.

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>