ath6kl: Implement support for background scan control from userspace

In order to allow user space based control of background scan interval,
we use available debugfs infrastructure. The feature has been added for
testing purposes. The user has to write the bgscan interval (in secs) to
the bgscan_interval file in ath6kl debug directory. To disable bgscan,
a '0' is to be written to the bgscan_interval file.

Example:

echo "2" > bgscan_interval

This will make the background scan interval as 2 seconds

kvalo: changed implementation so that there's only one call to
ath6kl_wmi_scanparams_cmd()

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