ixgbe: fix Tx error stats by setting it to 0

ixgbe: fix Tx error stats by setting it to 0

oerrors was txdgpc - hw_stats->gptc,
txdgpc is the number of packets DMA'ed by the host
and was being reset on every call to read stats so it could be < gptc.
Because we currently have no way to add txdgpc to struct hw_stats so
that we can maintain a persistent value per port oerrors has now been
set to 0. References to txdgpc is now removed as we don't use it. This
patch also removes rxnfgpc as it's not used anywhere.

Fixes: afebc86be134 ("ixgbe: refactor stats register reads")

Signed-off-by: Maryam Tahhan <maryam.tahhan@intel.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>