iwlwifi: allow to switch to HT40 if not associated

My previous patch
34a5b4b6

 iwlwifi: do not re-configure
HT40 after associated

Fix the case of HT40 after association on specified AP, but it break the
association for some APs and cause not able to establish connection.
We need to address HT40 before and after addociation.

CC: stable@vger.kernel.org #3.0+
Reported-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Tested-by: Andrej Gelenberg <andrej.gelenberg@udo.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>