ath10k: protect src_ring state with ce_lock in tx_sg()

It was possible to read invalid state of CE ring
buffer indexes. This could lead to scatter-gather
transfer failure in mid-way and crash firmware
later by leaving garbage data on the ring.

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>