cfg80211: fix wext-compat memory leak

cfg80211_wext_giwrate and sinfo.pertid might allocate sinfo.pertid via
rdev_get_station(), but never release it. Fix that.

Fixes: 8689c051 ("cfg80211: dynamically allocate per-tid stats for station info")
Signed-off-by: Stefan Seyfried <seife+kernel@b1-systems.com>
[johannes: fix error path, use cfg80211_sinfo_release_content(), add Fixes]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>