mac80211: manage RX BA session offload without SKB queue

Instead of using the SKB queue with the fake pkt_type for the
offloaded RX BA session management, also handle this with the
normal aggregation state machine worker. This also makes the
use of this more reliable since it gets rid of the allocation
of the fake skb.

Combined with the previous patch, this finally allows us to
get rid of the pkt_type hack entirely, so do that as well.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>