[MAC80211]: fix bug for per-sta stats

pre_rx handlers can't really touch sta since for IBSS it might not be
assigned yet, it can create sta info structs on-the-fly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>