net/mlx4_core: Check port out-of-range before using in mlx4_slave_cap

The range check was performed after using the port number.

Reverse this to prevent a potential array overflow.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>