ath6kl: support rssi threshold for sched scan

The ath6kl firmware can filter scan results based on rssi. This is
useful to limit hosts wakeups on scheduled scans.

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