iwlagn: Enable idle powersave mode in 1000 series

The iwlagn powersave algorithm uses the supports_idle parameter
to tell the device to save power when it is not associated with
an AP and is idle.

Enable this feature for the 1000 series of devices.

Reported-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>