ath10k: fix handling of wierd MSDU chaining cases

Apparently firmware can sometimes report a
sequence with the first rx descriptor saying it's
not the last MSDU. In that case msdu_chaining
value could be overwritten saying it's not a
chained MSDU. This in turn led to skb_push panic
as the frame could be treated as an A-MSDU instead
of a chained MSDU.

Reported-By: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>