net: mvneta: fix trivial cut-off issue in mvneta_ethtool_update_stats

When s->type is T_REG_64, the high 32bits are lost in val. This patch
fixes this trivial issue.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Fixes: 9b0cdefa ("net: mvneta: add ethtool statistics")
Signed-off-by: David S. Miller <davem@davemloft.net>