mac80211_hwsim: calculate if_combination.max_interfaces

If we just set this to 2048, and have multiple limits you
can select from, the total number might run over and cause
a warning in cfg80211. This doesn't make sense, so we just
calculate the total max_interfaces now.

Reported-by:  <syzbot+8f91bd563bbff230d0ee@syzkaller.appspotmail.com>
Fixes: 99e3a44b

 ("mac80211_hwsim: allow setting iftype support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>