mac80211_hwsim: Fill timestamp beacon at the time it is transmitted

Generate more acurate tsf values in hwsim by setting the tsf value on
trasmitted beacons immediately before they are moved to the rx path.
Also, adjust the beacon timestamp to be the time at which the first byte
of the timestamp is transmitted.

With these changes the observed tsf offset between two hwsim/mesh peers
is 0 (unless the offset is modified via debugfs)

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>