[MAC80211]: improve key selection comment

When I changed the code there I forgot to mention what happens
with multicast frames in a regular BSS and keep wondering myself
if the code is correct. Add appropriate comments.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Michael Wu <flamingice@sourmilk.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>