ixgbe: Stop cacheing if the MNG FW enabled

We use to cache whether the MNG FW was enabled, how since this isn't
static we really need to verify with each check.  This patch makes that
change.

CC: Arun Sharma <asharma@fb.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>