ath10k: register ath10k_htt_htc_t2h_msg_handler

Except qca61x4 family chips (qca6164, qca6174), copy engine 5 is used
for receiving target to host htt messages. In follow up patch, CE5
descriptors will be reused. In such case, same API can not be used as
htc layer callback where the response messages will be freed at the end.
Hence register new API for HTC layer that free up received message and
keep the message handler common for both HTC and HIF layers.

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>