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: 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>