b43(legacy): include full timestamp in beacon frames

Having the full RX timestamp in beacons is necessary for IBSS
merge to work properly so extend the 16-bit timestamp to the
full 64 bits for beacon frames (as well as when monitor mode
is active.)

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>