Update scan interval gracefully

Update scan interval gracefully

When the scan interval is changed the new interval is effective
after the old interval timer fires off one last time. This can cause
an unacceptable long delay when updating the interval.

Change this behaviour to use MIN(left of old interval, new interval)
for the scan interval following the interval change.

Signed-hostap: Pontus Fuchs <pontus.fuchs@gmail.com>