mlx4_core: Fix fallback from MSI-X to INTx

The test in mlx4_load_one() to remove MLX4_FLAG_MSI_X expects mlx4_NOP() to
fail with -EBUSY. It is also necessary to avoid the reset since the device
is not fully reinitialized before calling mlx4_start_hca() a second time.

Note that this will also affect mlx4_test_interrupts(), the only other user
of MLX4_CMD_NOP.

Fixes: f5aef5aa

 ("net/mlx4_core: Activate reset flow upon fatal command cases")
Signed-off-by: Benjamin Poirier <bpoirier@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>