wl12xx: minor fix in sched_scan_ssid_list

The user can pass broadcast SSID (ssid="") in the list of SSIDs for active scan.
In this case the loop was attempting to match SSIDs in the filter
list to this empty entry and marking them as HIDDEN (sending probe
request) by mistake

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>