ixgb: cleanup link up/down messages

The system log messages created on a link status change need to follow a
specific format to work with tools some customers use.  This also makes
the messages consistant with other Intel driver link messages.

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>