ath10k: export htt tx rx handlers

Some special copy engines delivers messages directly to HTT by
bypassing HTC layer. Hence exporting tx_completion and rx_handler
for delivering the data to HTT layer.

Reviewed-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>