wl1271: Move scan complete invocation into work function

The current scan implementation can jam, if the scan request ends up
containing no work. This can especially happen if there is a scan request
with only 11a band channels for HW that does not support 11a.

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