iwlwifi: op-mode API: add rx_rss method

Upcoming hardware will have the ability to do L3 hashing for RSS,
directing data packets (and perhaps some associated metadata and
management notifications) to different MSI-X vectors.

In this case, it makes no sense to go through the full RX dispatch
since it's already known that only a subset of the possibilities
can come in, requiring a new receive method. In addition this must
know which queue the packet was received on.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>