ixgbe: enable multicast on shutdown for WOL

Previously we only enabled the reception of multicast packets when
wake on multicast is set, but we also need this to allow waking with
IPv6 magic packets.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>