ath6kl: Implement support for listen interval from userspace

In order to allow user space based control of listen interval, we use
available debugfs infrastructure. Listen interval implies how frequently
we want the WLAN chip to wake up and synchronize the beacons in case it
is in sleep mode. The command requires two parameters in the following order:

1) listen_interval_time
2) listen_interval_beacons

The user has to write the listen interval_time (in msecs) and
listen_interval_beacons (in no. of beacons) to the listen_interval file in
ath6kl debug directory.

Example:

echo "30 1" > listen_interval

Signed-off-by: Rishi Panjwani <rpanjwan@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>