iwlwifi: Add chain noise scaling factor

6x50 device requires a different scaling factor for Rx gain values sent to
device via PHY_CALIBRATION_CMD (CHAIN_NOISE_GAIN_CMD).  Rather than create
a new iwlXXXX_gain_computation() function, add new chain_noise_scale member
to struct iwl_cfg, and keep using iwl5000_gain_computation().

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>