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>