iwlwifi: fix gain computation for 5000 series and up

In Rx gain balancing (chain noise) computation for 5000 series and up,
the delta gain calculation should use the average noise of default
chain, not "chain 0" which do not exist for all the devices.
Signed-off-by: NWey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: NReinette Chatre <reinette.chatre@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-5000.c

@@ -236,7 +236,7 @@ static void iwl5000_gain_computation(struct iwl_priv *priv,
 			data->delta_gain_code[i] = 0;
 			continue;
 		}
-		delta_g = (1000 * ((s32)average_noise[0] -
+		delta_g = (1000 * ((s32)average_noise[default_chain] -
 			(s32)average_noise[i])) / 1500;
 		/* bound gain by 2 bits value max, 3rd bit is sign */
 		data->delta_gain_code[i] =