iwlwifi: mvm: add minimal multi-RXQ infrastructure

Since the new multi-queue capability depends on a new firmware API,
we can already add some code for it. If the new API is present, a
new opmode ops struct is used that handles the new rx_rss method.

For now, only restructure the RX handling to distinguish between
the two. Future patches will convert the new infrastructure to
actually use the new RX descriptor layout.

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