net/mlx5_core: Check the return bitmask when querying ISSI

The determination of the supported ISSI versions should be conditioned
on the returned mask, and not only on the return status of the query
ISSI command, fix that.

Signed-off-by: Haggai Abramovsky <hagaya@mellanox.com>
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>