net/mlx4_en: Split SW RX dropped counter per RX ring

Count SW packet drops per RX ring instead of a global counter. This
will allow monitoring the number of rx drops per ring.

In addition, SW rx_dropped counter was overwritten by HW rx_dropped
counter, sum both of them instead to show the accurate value.

Fixes: a3333b35 ('net/mlx4_en: Moderate ethtool callback to [...] ')
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reported-by: Brenden Blanco <bblanco@plumgrid.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>