ath10k: introduce wmi ops

Since the 10.x fw branch support was introduced it
became apparent ath10k will need to be able to
deal with different fw ABIs eventually.

The patch creates an abstraction for dealing with
command and event structures across different ABIs
and mostly gets rid of the
ATH10K_FW_FEATURE_WMI_10X flag usage.

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