iwlagn: 5000 do not support idle mode

5000 series has issue supporting power save idle mode:

commit	9dc21533

iwlwifi: always support idle mode for agn devices

For agn devices, always support idle mode which help power
consumption in idle unassociated state.

the above changes cause 5000 become not stable when power management is "on"

http://bugzilla.intellinuxwireless.org/show_bug.cgi?id=2312



Cc: stable@kernel.org #2.6.39, #3.0.0
Reported-by: Devin J Pohly <djpohly+iwl@gmail.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>