net/mlx4_en: Count HW buffer overrun only once

RdropOvflw counts overrun of HW buffer, therefore should
be used for rx_fifo_errors only.

Currently RdropOvflw counter is mistakenly also set into
rx_missed_errors and rx_over_errors too, which makes the
device total dropped packets accounting to show wrong results.

Fix that. Use it for rx_fifo_errors only.

Fixes: c27a02cd

 ('mlx4_en: Add driver for Mellanox ConnectX 10GbE NIC')
Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>