cfg80211: simplify cfg80211_can_beacon_sec_chan API

Change cfg80211_can_beacon_sec_chan() to return true
if there is no secondary channel to simplify all the
current users of it. They all check the channel type
before calling the function because it returns false
if there's no secondary channel.

Also actually document the return value.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>