i40e: fix stats reset

This patch fixes a bug where only some of the statistics were being
reset when calling rte_eth_stats_reset() or rte_eth_xstats_reset().
As both the stats reset and xstats reset do the same, refactor away
the duplicated function.

This patch marks the VSI to update its offset, causing the stats be
look like they are reset.

Fixes: 9aace75fc82e ("i40e: fix statistics")

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Maryam Tahhan <maryam.tahhan@intel.com>