iwlwifi: 802.11n spec removes AUTO offset for FAT channel

This patch adapts to 802.11 patch and remove AUTO offset for FAT
channel

Signed-off-by: Guy Cohen <guy.cohen@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>