{cfg,mac}80211.h: fix some kernel-doc warnings

When building the 80211 DocBook, scripts/kernel-doc reports
the following type of warnings:

Warning(include/net/cfg80211.h:334): No description found for return value of 'cfg80211_get_chandef_type'

These warnings are only reported when scripts/kernel-doc
runs in verbose mode.

To fix these use "Return:" to describe function return values.

Signed-off-by: Yacine Belkadi <yacine.belkadi.1@gmail.com>
[adjust for freq_reg_info() change]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>