igb: expose mailbox unlock method

Add a mailbox unlock method to e1000_mbx_operations, which will be used
to unlock the PF/VF mailbox by the PF.

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>