mac80211_hwsim: require at least one channel

Syzbot continues to try to create mac80211_hwsim radios, and
manages to pass parameters that are later checked with WARN_ON
in cfg80211 - catch another one in hwsim directly.

Reported-by:  <syzbot+2a12f11c306afe871c1f@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>