mac80211_hwsim: fix use after free

Once the "data" pointer is freed, we can't be iterating
to the next item in the list any more so we need to use
list_for_each_entry_safe with a temporary variable.

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