When there are multiple RX queues, the PN checks in mac80211 cannot be
used since packets might be processed out of order on different CPUs.

Allow the driver to report that the PN has been checked, drivers that
will use multi-queue RX will have to set this flag.

For now, the flag is only valid when the frame has been decrypted, in
theory that restriction doesn't have to be there, but in practice the
hardware will have decrypted the frame already.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>