ath6kl: Fix listen interval handling

This patch addresses a few problems with the commit:

"ath6kl: Implement support for listen interval from userspace"

* The debugfs file required for reading/writing the listen interval
  wasn't created. Fix this.

* The interface index was being hardcoded to zero. Fix this.

* Two separate parameters, "listen_interval_time and listen_interval_beacons"
  were being used. This fails to work as expected because the FW assigns
  higher precedence to "listen_interval_beacons" and "listen_interval_time"
  ends up being never used at all.

  To handle this, fix the host driver to exclusively use listen interval
  based on units of beacon intervals.

To set the listen interval, a user would now do something like this:

echo "10" > /sys/kernel/debug/ieee80211/*/ath6kl/listen_interval

kvalo: fix two checkpatch warnings

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>