wl1251: Handle failing PS entry

wl1251: Handle failing PS entry

It is possible, that the nullfunc message used to indicate to the
associated-to AP that we are going to power save fails to reach the
intended recipient. In that case, the STA and AP can become out of sync,
resulting in connection loss.

This patch adds code to attempt to recover from such failure by
resending the nullfunc.

Also fixed one compiler warning.

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