iwlwifi: mvm: implement new IPv6 offload API

The firmware API for IPv6 NDP/NS offload has changed again.
Implement support for the new API; this requires calculating
the solicited node address for each "target" address as it's
no longer ignored by the firmware.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>