Have ibv_modify_qp() return right value when ibv_query_port() fails

When the ibv_query_port() call made by mlx4_modify_qp() fails, the
return value from the latter should indicate the error status of the
former and not simply -1.

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