iwlwifi: disable PS by default

Unfortunately, PS currently affects RX performance
significantly enough to warrant disabling it by
default, but give the user the choice to enable it
again with iwconfig.

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