cfg80211: add netlink channel put helper

This adds a netlink channel put helper, nl80211_msg_put_channel(),
which we will also make use of later for the beacon hints events.

Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>