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. == WIL6210_MAX_CID).

Change-Id: I47c6c185dead561180742608d9ac318e70ed7bd2
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Lior David <liord@codeaurora.org>