wl12xx: lock the RCU when accessing sta via ieee80211_find_sta()

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: Jonathan DE CESCO <jonathanc@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>