ath10k: add new FW_FEATURE_PEER_FLOW_CONTROL

This feature flag will be used for firmware which
supports pull-push model where host shares it's
software queue state with firmware and firmware
generates fetch requests telling host which queues
to dequeue tx from.

Primary function of this is improved MU-MIMO
performance with multiple clients.

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