IB/mlx4: Fix shutdown crash accessing a non-existent bitmap

Commit cfcde11c ("IB/mlx4: Use flow counters on IBoE ports") added
code that sets elements of counters[] to -1 if no counter is allocated,
but then goes ahead and passes every entry to mlx4_counter_free() on
shutdown.  This is a bad idea, especially if MLX4_DEV_CAP_FLAG_COUNTERS
isn't set so there isn't even an underlying bitmap to free from.

Tested-by: Sean Hefty <sean.hefty@intel.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>