mac80211: document return codes from ops callbacks

For any callbacks in ieee80211_ops, specify what values the return
codes represent.  While at it, fix a couple of capitalization and
punctuation differences.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Reviewed-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>