i40e: fix packet count stats

i40e: fix packet count stats

The statistics queried by calling rte_eth_stats_get are zero when
the API is first called on the port. The root cause is because the
offset_loaded flag is not set correctly after device start.
This patch fixes this issue by resetting statistics at initialization
time. The resetting process will set offset_loaded flag.

Fixes: 4861cde46116 ("i40e: new poll mode driver")
Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>