net: mvneta: discriminate error cause for missed packet

In order to improve the diagnostic in case of error, make the distinction
between refill error and skb allocation error. Also make the information
available through the ethtool state.

Based on the work of Yelena Krivosheev <yelena@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>