iwlwifi: align tx/rx statistics debugfs format

Align the format for tx_statistics and rx_statistics debugfs output for
better readability

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>