mac80211: Determine dynamic PS timeout based on ps-qos network latency

Determine the dynamic PS timeout based on the configured ps-qos network
latency. For backwards wext compatibility, allow the dynamic PS timeout
configured by the cfg80211 to overrule the automatically determined value.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>