ath6kl: Fix bug in bg scan configuration in schedule scan

Background scan interval should not be modified while starting
schedule scanning as it changes the bg scan interval when connected to AP.
Use the currently configured interval instead.

kvalo: improve commit log

Signed-off-by: Subramania Sharma <sharmat@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>