wil6210: fix invalid sta statistics update

Upon status ring handling, in case there are both unicast and
multicast (cid == max) status messages to handle, wrong sta statistics
might get updated.
Fix this by setting stats to NULL upon invalid cid
(e.g. == max_assoc_sta).

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>