ath6kl: Configure inactivity timeout in fw

Configure the inactivity timeout passed in start_ap() to
firmware. This capability is advertised only when fw supports
it, there is a new bit (ATH6KL_FW_CAPABILITY_INACTIVITY_TIMEOUT)
in firmware capability ie for driver to learn fw's capability.
After the fw finds out the station is inactive, it will probe
the station with null func frames. By default, the timeout is
10 secs.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>