mac80211_hwsim: fix tsf adjustment

Make hwsim TSF offset adjustments cumulative and relative
to the fake PHY TSF. Now adding 1000us, then adding -1000us
doesn't result in a tsf_offset of -1000. Also the beacon
timestamp can now correctly be expressed as (tsf +
data->tsf_offset), which will be done in a later patch.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>