iwlwifi: control 11n capabilities through module param

This patch adds module param 11n_disable to allow configuration of 11n
capabilities. The default value of this param is 11n enabled (value 0).

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>