mac80211: improve RX aggregation data in debugfs

When the driver sets the SUPPORTS_REORDERING_BUFFER hardware flag,
the debugfs data for RX aggregation sessions won't even indicate
that a session is open. Since the previous fix to store the dialog
token separately, we can indicate that it's open and add the token
so that there's at least some data (ssn is not available.)

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