iwlwifi: format and show statistics counter from uCode

To help debug uCode related problem, adding "delta" and "max"
information in debugfs statistics counters display.

Those information show the delta between two statistics report from
uCode, user can monitor the counters for any "un-normal" behavior.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Jay Sternberg<jay.e.sternberg@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>