mac80211: allow controlling aggregation manually

This allows enabling TX and disabling both TX and
RX aggregation sessions manually in debugfs. It is
very useful for debugging session initiation and
teardown problems since with this you don't have
to force a lot of traffic to get aggregation and
thus have less data to analyse.

Also, to debug mac80211 code itself, make hwsim
"support" aggregation sessions. It will still just
transfer the frame, but go through the setup and
teardown handshakes.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>