We were calling ieee80211_find_sta() and the sta returned by it
without locking the RCU, which is required by mac80211.

Fix this and reorganize slightly the area of the code where the sta is
used.
Reported-by: NJonathan DE CESCO <jonathanc@ti.com>
Signed-off-by: NLuciano Coelho <coelho@ti.com>