ath6kl: support TX error rate notification

The ath6kl firmware can monitor a connection and report when a certain
TX failure threshold is crossed. Support this configuration and event
reporting on compatible firmwares.

Signed-off-by: Thomas Pedersen <c_tpeder@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>