mac80211_hwsim: fix beacon delta calculation

Due to the cast from uint32_t to int64_t, a wrong next beacon timing is
calculated and effectively the beacon timer stops working. This is
especially bad for 802.11s mesh networks, because discovery breaks
without beacons.

Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>