wpa_supplicant: fix scan with more than max_sched_scan_ssids

sched scan fills filter_ssids with non empty ssids
that may be non-hidden (not scan_ssid=1)
so we may get to a point where some hidden ssids aren't filtered
and we don't start a normal scan for them, since
total number of hidden ssid < max_sched_scan_ssids.
to solve this, we count all active ssids instead of just the
scan_ssid=1 ones