net/mlx5: fix probing if DevX disabled

net/mlx5: fix probing if DevX disabled

If there is the support of DevX is exposed by rdma-core but
DevX is not supported by or disabled for the specific interface
the mlx5_devx_cmd_query_hca_attr() routine returns an error
preventing the device from successful probing. The routine
should be invoked only in case of enabled DevX.

Fixes: e2b4925ef7c1 ("net/mlx5: support Direct Rules E-Switch")
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>