mac80211_hwsim: Add tsf to beacons, probe responses and radiotap header.

Generate a tsf from internal kernel clock.  Prepare the path for having
different tsf offsets on each phy.  This will be useful for testing
mesh synchronization algorithms.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>