iwlwifi: fix clear statistics counter command

When receive reply statistics command with "clear" mask, just reset the
accumulated statistics counters, but not the current statistics counters,
so the accumulated statistics counter can provide the correct
information.

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