wl1271: Reduce rate used for last PSM entry attempt

This patch reduces the rate of the null-func used to enter PSM on the last
retry as precaution.

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>