wl12xx: add support for multiple SSIDs in sched_scan

The wl12xx firmwares support multiple SSIDs in a single sched_scan
run.  This patch implements support for it.

We use three different types os sched_scan: FILTER_ANY (ie. not
filtering, only wildcard SSID in the probe_reqs); FILTER_LIST (ie. send out
probe_reqs with the specified SSIDs and only report if they are
found); and FILTER_DISABLED (ie. send out probe_reqs with the
specified SSIDs, but report anything found).

Since we still don't have proper filter support in nl80211/cfg80211
yet, we cannot use filters when the wildcard SSID is used.  Thus, we
will not filter anything if the wildcard SSID is specified.

Signed-off-by: Luciano Coelho <coelho@ti.com>