[net80211] put in a comment about the not-quite-correctness of A-MPDU parameters.

Although we correctly (now!) calculate the right A-MPDU parameters, the
ioctl() has some faulty logic for choosing which to display. The BSS
params are what were advertised to us, and we would have chosen the
lower of theirs/ours when advertising the HT bits back at them.

So, we /should/ track and fix that so we display the correct A-MPDU
density and size.

However, since I'm a forgetful type, and I don't want to have to re-learn
that this is wrong, drop in a comment so I or someone else fixes it.
Or, when I discover this again in 4 years, I don't have to go digging
too much to remember.