cfg80211: allow driver to override PS default

Sometimes drivers might have a good reason to override
the PS default, like iwlwifi right now where it affects
RX performance significantly at this point. This will
allow them to override the default, if desired, in a
way that users can still change it according to their
trade-off choices, not the driver's, like would happen
if the driver just disabled PS completely then.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>