mac80211: add docs for ssn in struct tid_ampdu_tx

As pointed out by Stephen, add the missing docs.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20211129091948.1327ec82beab.Iecc5975406a3028d35c65ff8d2dec31a693888d3@changeid


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