wifi: mac80211_hwsim: add back erroneously removed cast

The robots report that we're now casting to a differently
sized integer, which is correct, and the previous patch
had erroneously removed it.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 4ee186fa

 ("wifi: mac80211_hwsim: fix race condition in pending packet")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>