iwlagn: set smps mode after assoc for 1000 device

For some timing reason, 1000 device having problem to kick-in to aggregation
without sending rxon assoc command. This is a W/A until find the real reason

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>