iwlwifi: fix comment describing disable_11n

Fixing a comment in the code describing this module parameter.
The description printed when user runs "modinfo" is correct.

Reported-by: Jiajia Zheng <jiajia.zheng@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>