ath6kl: add htc ops

In preparation for adding HTC pipe implementation add htc-ops.h to make
it possible dynamically choose which HTC type is used.

Needed for full USB support.

Based on the code by Ray Chen <raychen@qca.qualcomm.com>.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Ray Chen <raychen@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>