ixgbe: Add work around for empty SFP+ cage crosstalk

It is possible on some systems that crosstalk could lead to link flap
on empty SFP+ cages.  A new NVM bit was defined to let SW know it
needs to implement the work around which consists of verifying that
there is a module in the cage before acting on the LSC.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>