ixgbe: disable link when adapter goes down

This patch fixes an issue with the 82599 adapter where it can potentially keep
link lights up when the adapter has gone down. The patch adds a function which
ensures link is disabled, and calls this function when the adapter transitions
to a down state.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>