ixgbe: warn when device is in a x4 or lower width slot

It's easy to oversee this issue when working with this card
as evrything will work OK but performance is severely limited
(something like 1.5gbit on a x1 link) if the pci-express
slot does not offer more bandwidth.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>