igb: constify e1000_phy_operations structure

This e1000_phy_operations structure is never modified, so declare it as
const.  Other structures of this type are already const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>