net: mvneta: move refill_err and skb_alloc_err in per-cpu stats

mvneta_ethtool_update_stats routine is currently reporting
skb_alloc_error and refill_error only for the first rx queue.
Fix the issue moving skb_alloc_err and refill_err in
mvneta_pcpu_stats structure.
Moreover this patch will be used to introduce xdp statistics
to ethtool for the mvneta driver

Fixes: 17a96da6 ("net: mvneta: discriminate error cause for missed packet")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>