cfg80211: remove radar requirements check from cfg80211_can_use_iftype_chan()

We don't have to double check whether the parameters passed to
cfg80211_can_use_iftype_chan() are correct.  We should just make sure
they *are* when we call this function.

Remove the radar_detect argument check in
cfg80211_can_use_iftype_chan() to simplify the code.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
[keep braces around a long comment + single statement]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>