ath10k: add inline wrapper for htt_h2t_aggr_cfg_msg

This is done in order to make the *htt_h2t_aggr_cfg_msg* op align better
with the rest of the htt ops (whom all have inline wrappers).

It also adds support for the case when the op is missing (function
pointer is NULL).

As a result of this, the name of the 32 bit implementation in htt_tx.c
was changed and the function was made static.

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