ixgbe: fix inconsistent clearing of the multicast table

This patch resolves an issue where the MTA table can be cleared when the
interface is reset while in promisc mode. As result IPv6 traffic between
VFs will be interrupted.

This patch makes the update of the MTA table unconditional to avoid the
inconsistent clearing on reset.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>