wl1251: don't toggle PS, let firmware manage it

This enables dynamic PS in mac80211, which stops it toggling PS based on
network activity and (mostly) keeps it on or off based on the user setting.
This is how the TI driver operates too, which enables PS in firmware and
never touches the setting after that.

Without this I'm constantly getting SYNCHRONIZATION_TIMEOUT events with
my current router. Doing it this way also makes the device accessible
from network much better with much lower ping times.

At least with my current router, PS entry must be severely delayed too
to avoid repeated sync timeouts. This patch adds such delays too.