wl12xx: add forced_ps mode

For certain WiFi certification tests forcing PS
is necessary. Since DPS is now enabled in the FW
and this can't be achieved by using netlatency
this required a new config option.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>