be2net: fix netdev_stats_update

Problem initially reproted and fixed by Eric Dumazet <eric.dumazet@gmail.com>

netdev_stats_update() resets netdev->stats and then accumulates stats from
various rings. This is wrong as stats readers can sometimes catch zero values.
Use temporary variables instead for accumulating per-ring values.

Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>