net/mlx5: E-Switch, Fix out of bound access when setting vport rate

The code that deals with eswitch vport bw guarantee was going beyond the
eswitch vport array limit, fix that.  This was pointed out by the kernel
address sanitizer (KASAN).

The error from KASAN log:
[2018-09-15 15:04:45] BUG: KASAN: slab-out-of-bounds in
mlx5_eswitch_set_vport_rate+0x8c1/0xae0 [mlx5_core]

Fixes: c9497c98 ("net/mlx5: Add support for setting VF min rate")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>