nl80211: fix dump callbacks

Julius Volz pointed out that the dump callbacks in nl80211 were
broken and fixed one of them. This patch fixes the other three
and also addresses the TODOs there.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Julius Volz <juliusv@google.com>
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: John W. Linville <linville@tuxdriver.com>