iwlwifi: enable receiving beacons when not associated

If the firmware implements beacon filtering,
beacons are filtered when not associated. This causes
association failures on channels marked passive.

Enabling this flag indicates to the firmware to allow
the beacons to pass through when not associated only.

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