iwlwifi: set auto clock gate disable bit for 6x00/6x50 series

For 6x00 and 6x50 series NIC with OTP shadow RAM, set auto clock gate
disable bit when initializing OTP access.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>