ethtool, net/mlx4_en: Add 100M, 20G, 56G speeds ethtool reporting support

Added 100M, 20G and 56G ethtool speed reporting support.
Update mlx4_en_test_speed self test with the new speeds.

Defined new link speeds in include/uapi/linux/ethtool.h:
+#define SPEED_20000	20000
+#define SPEED_40000	40000
+#define SPEED_56000	56000

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>