wcn36xx: Fix dynamic power saving

Since driver does not report hardware dynamic power saving cap,
this is up to the mac80211 to manage power saving timeout and
state machine, using the ieee80211 config callback to report
PS changes. This patch enables/disables PS mode according to
the new configuration.

Remove old behaviour enabling PS mode in a static way, this make
the device unusable when power save is enabled since device is
forced to PS regardless RX/TX traffic.

Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>