net/i40e: fix corruption of VF stats

net/i40e: fix corruption of VF stats

The resetting of stats on a VF involves the setting of an offset
that is subtracted from future calls, rather zeroing of counters.
However doing a stats reset on the port representor was also
adjusting the values forwarded to the VF, which had the effect of
corrupting the VF's counters. The fix is for the port representor
to maintain its own stats offset, so the port representor and VF
maintain independently-resettable counters.

Fixes: e0cb96204b71 ("net/i40e: add support for representor ports")

Signed-off-by: Remy Horton <remy.horton@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>