The current implementation of handling ADDBA Request while a session
is already active with the peer is wrong - in case the peer is using
the existing session's dialog token this should be treated as update
to the session, which can update the timeout value.

We don't really have a good way of supporting that, so reject, but
implement the required behaviour in the spec of "Even if the updated
ADDBA Request frame is not accepted, the original Block ACK setup
remains active." (802.11-2012 10.5.4)
Signed-off-by: Johannes Berg <johannes.berg@intel.com>