mlx4: trust shinfo->gso_segs

mlx4 is the only driver in the tree making a point to recompute
shinfo->gso_segs.

Lets remove superfluous code.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>