igb: add link check function

Add a link check function to contain all activities related to verifying
that the link is present.  The current approach is a bit cludgy and needs
to be cleaned up.

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>