net/mlx4_en: Fix set port ratelimit for 40GE

In 40GE we can't use the default bw units for set ratelimit (100 Mbps)
since the max is 255*100 Mbps = 25 Gbps (not suited for 40GE), thus we need 1 Gbps units.
But for 10GE 1 Gbps units might be too bruit so we use the following solution.

For user set ratelimit <= 25 Gbps:
        use 100 Mbps units * user_ratelimit (* 10).

For user set ratelimit > 25 Gbps:
        use 1 Gbps units * user_ratelimit.

For user set unlimited ratelimit (0 Gbps):
        use 1 Gbps units * MAX_RATELIMIT_DEFAULT (57)

Note: any value > 58 will damage the FW ratelimit computation, so we allow
      a max and any higher value will be pulled down to 57.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>