IB/mlx4: Diagnostic HW counters are not supported in slave mode

Modify the mlx4_ib_diag_counters() to avoid the following error in the
hypervisor when the slave tries to query the hardware counters in SR-IOV
mode.

mlx4_core 0000:81:00.0: Unknown command:0x30 accepted from slave:1

Fixes: 3f85f2aa ("IB/mlx4: Add diagnostic hardware counters")
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/hw/mlx4/main.c

@@ -2202,6 +2202,9 @@ static int mlx4_ib_alloc_diag_counters(struct mlx4_ib_dev *ibdev)
 	bool per_port = !!(ibdev->dev->caps.flags2 &
 		MLX4_DEV_CAP_FLAG2_DIAG_PER_PORT);
 
+	if (mlx4_is_slave(ibdev->dev))
+		return 0;
+
 	for (i = 0; i < MLX4_DIAG_COUNTERS_TYPES; i++) {
 		/* i == 1 means we are building port counters */
 		if (i && !per_port)