ath10k: add initialization of HTC header

HTC header carries junk values that may be interpreted by the firmware
differently. Enable credit update only if flow control is enabled for
the corresponding endpoint.

Signed-off-by: Alagu Sankar <alagusankar@silex-india.com>
Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>