wl1271: Fix scan failure detection

In scan_complete_work, because the mutex is released before accessing the
scan->failed flag, it is possible for unfounded hardware recovery rounds
to be executed.

Fix this.

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>