mac80211_hwsim: check that n_limits makes sense

Under certain circumstances, radios created via netlink could
have n_limits be zero and no possible interface types, which
makes no sense. Reject this early to prevent a WARN_ON() in
cfg80211.

Fixes: 99e3a44b

 ("mac80211_hwsim: allow setting iftype support")
Reported-by:  <syzbot+73fd8b0aa60c67fa4b60@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>