iwlwifi: enhance WPA authenication stability

This patch enhanced WPA authenication stability by avoiding scan
immediately followed by association. We don't do any scanning right
after association in next several seconds. This will allow WPA
authentication to take place without any interruption.

Signed-off-by: Mohamed Abbas <mabbas@linux.intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>