ixgbe: fix ixgbe_stop_mac_link_on_d3_82599 to check mng correctly

Previously, we did a full check to see if MNG FW was running. Instead,
we should only check to see whether it could be enabled. Since it may
become active while down, we don't want to bring the link down.

CC: Arun Sharma <asharma@fb.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
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>