rsi: missing unlocks on error paths

There is a missing unlock if rsi_find_sta() fails in
rsi_mac80211_ampdu_action() or if we hit the -EINVAL path in
rsi_mac80211_sta_add().

Fixes: 3528608f ("rsi: handle station connection in AP mode")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>