IB/mlx4: Fill the masked_atomic_cap attribute in query device

When the user queries for device capabilities, fill in the
masked_atomic_cap attribute with the real support level of atomic
capabilities instead of using a hard coded value.

Signed-off-by: Dotan Barak <dotanb@dev.mellanox.co.il>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>