[PATCH] fix wrong comments in ieee80211.h

The comments in ieee80211.h claim that one doesn't need to set the len
parameter of the stats struct. But if one doesn't, the management frames
are read far over the memory they actually occupy causing badness.

Signed-Off-By: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>