mac80211: Don't request encryption for probe response

Probe responses shouldn't be encrypted, and mac80211 doesn't
set the crypto key accordingly. However it didn't set the
IEEE80211_TX_CTL_DO_NOT_ENCRYPT flag which means drivers
could make an attempt to encrypt it, and causing a NULL
pointer dereference when accessing the provided hw_key field.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>