mac80211_hwsim: validate number of different channels

When creating a new radio on the fly, hwsim allows this
to be done with an arbitrary number of channels, but
cfg80211 only supports a limited number of simultaneous
channels, leading to a warning.

Fix this by validating the number - this requires moving
the define for the maximum out to a visible header file.

Reported-by:  <syzbot+8dd9051ff19940290931@syzkaller.appspotmail.com>
Fixes: b59ec8dd ("mac80211_hwsim: fix number of channels in interface combinations")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>