IB/mlx5: Avoid async events on invalid port number

On a single ported Connect-IB, its possible for the firmware to issue
events on the non-existing 2nd port.  Make sure to ignore events
generated for such ports.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>