wl1271: fix forced passive scan

When we get no SSIDs in the scan request, we should force a passive scan in
all channels.  This patch adds code to force the passive scan flag to be set
in that case.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>