wl1271: Increase connection reliability

This patch improves connection reliability by choosing the lowest basic
rate for null-func frames (which increases their range, as the firmware does
not do rate fall-back for null-func frames.) Also, increase the PSM entry
retry-counter.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Tested-by: Tuomas Katila <ext-tuomas.2.katila@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>