ath10k: add inlined wrappers for htt tx ops

These wrappers makes the HTT ops align better with the HIF ops
(where similar wrappers are used).

It also makes it easier for a target to have unsupported ops
(by letting the corresponding function pointer be NULL).

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