mac80211: fix memory leak

On error we jumped to the error label and returned the error code but we
missed releasing sinfo.

Fixes: 5fe74014172d ("mac80211: avoid excessive stack usage in sta_info")
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>