wl12xx: set the ELP entry delay to the FW dyn-ps timeout

With PSM handled in FW, the checks in wl1271_elp_work() are always true.
Thus during active traffic we constantly enter and exit ELP (many times
per second). As each ELP exit takes ~10ms, this can have an adverse
effect on throughput and interactivity.
Set the ELP timeout to the dyn-ps timeout. This period is longer and
avoids the above problem. It also makes sense to stay out of ELP while
we are awake on the network, to minimize delays in Tx/Rx. The same thing
was done by the mac80211 dynamic-ps mechanism before the FW DPS changes.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>