nl80211: clean up genlmsg_end uses

genlmsg_end() cannot fail, it just returns the length
of the message. Thus, error handling for it is useless.
While removing it, I also noticed a useless variable
and removed this it as well.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>