ath6kl: Maintain the listen interval per VIF specific

Firmware has the option to support the listen interval
per vif specific. Fix this.

Listen interval can be set by the TUs or by the number
of beacons. Current code enables the user to configure
the listen interval in the unit of 'number of beacons'
using debugfs entry "listen_interval". Going forward,
we need to alter the listen interval in the unit of TUs
to get good power numbers while going to WOW suspend/resume.

Allowing the user to change the listen interval in
the unit of "number of beacons"  in debugfs and changing
listen interval in wow suspend/resume in the unit of
time (TUs) would lead us to confuse.

This patch make sures the listen interval is changed only
in the unit of time (TUs).

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>