ixgbe: fix multicast address update

after the most recent patches, the driver was not using the
correct iterator for updating the receive address registers (RAR)

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>