iwlwifi: reset led_tpt when clear tx/rx traffic byte counts

LED blink rate is based on the traffic load, when tx/rx traffic counts
got reset, we also need to reset the led_tpt to prevent incorrect
blink rate being calculated.

Merge both clear_tx_statistics() and clear_rx_statistics() into
single clear_traffic_statistics() function, when reset the traffic byte
counts, both tx and rx need to be reset at the same time, to make
sure calculated the correct led blink rate.

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>