mac80211: dereference RCU protected probe_resp pointer correctly

This fixes a sparse warning:

cfg.c:502:13: warning: incorrect type in assignment (different address spaces)
cfg.c:502:13:    expected struct sk_buff *old
cfg.c:502:13:    got struct sk_buff [noderef] <asn:4>*probe_resp

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>