net/igc: fix Rx error counter for bad length

net/igc: fix Rx error counter for bad length

When the size of a packet in Rx channel is less than the minimum
or greater than the maximum, the packet will be simultaneously
counted by RLEC(Receive Length Error Count) and
RUC(Receive Under Size Count)/ROC(Receive Oversize Count) registers.

This patch fixes the issue of counting a length error packet twice
when counting the total number of received error packets.

Fixes: e6defdfddc3b ("net/igc: enable statistics")
Cc: stable@dpdk.org
Signed-off-by: Alvin Zhang <alvinx.zhang@intel.com>
Acked-by: Haiyue Wang <haiyue.wang@intel.com>