mac80211: Comment the order of HT RX reorder handler vs. RX handlers

We are currently processing block ack reordering as a separate task
before all other RX handlers. In theory, this is wrong since this step
should be done only after duplicate removal (see Figure 6-1 in IEEE
802.11n). However, moving this needs some work and the current
situation is not too bad. Add a comment here so that this small detail
does not get forgotten and who knows, maybe someone has some extra
time to take a look at cleaning this up.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>