ath6kl: Add debugfs control for keepalive and disconnection timeout

The new debugfs files keepalive and disconnect_timeout can be used to
fetch the current values and to change the values for keepalive and
disconnect event timeout (both in seconds).

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>