ath10k: protect ath10k_htt_rx_ring_free with rx_ring.lock

While debugging driver crashes related to a buggy firmware
crashing under load, I noticed that ath10k_htt_rx_ring_free
could be called without being under lock.  I'm not sure if this
is the root cause of the crash or not, but it seems prudent to
protect it.

Originally tested on 4.16+ kernel with ath10k-ct 10.4 firmware
running on 9984 NIC.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>