igb: make ioport free

This patch makes igb driver ioport-free.
This corrects behavior in probe function so as not to request ioport
resources as long as they are not really needed.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>