mac80211: Don't encrypt beacons

mac80211 should set the IEEE80211_TX_CTL_DO_NOT_ENCRYPT flag in tx_control
structure to inform drivers not to encrypt the beacon. Drivers that only check
for that flag before accessing the hw_key field, will otherwise cause a NULL
pointer dereference since that field is not configured for beacons.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>