iwlwifi: change IWL6000_UCODE_API_MAX to v4

uCode version changed to v4 for 6000 series

The additional parameter added to v4 is providing current tx power for
each chain in tx statistics portion of "statistics notification"
command.

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>