nl80211: Fix documentation errors

Couple of '_ATTR's were missing and SEC_CHAN_OFFSET to CHANNEL_TYPE
rename was missed in couple of places.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>