net/mlx4_en: Fix off-by-one in ethtool statistics display

NUM_PORT_STATS was 9 instead of 10, which caused off-by-one bug when
displaying the statistics starting from tx_chksum_offload in ethtool.

Fixes: f8c6455b ('net/mlx4_en: Extend checksum offloading by CHECKSUM COMPLETE')
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>