net: mvneta: rely on struct mvneta_stats in mvneta_update_stats routine

Introduce mvneta_stats structure in mvneta_update_stats routine signature
in order to collect all the rx stats and update them at the end at the
napi loop. mvneta_stats will be reused adding xdp statistics support to
ethtool.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>