mlx5core: Make sure error code is propagated on error.

If mlx5_init_once() fails, mlx5_load_one() should fail too, else the
device instance remains attached causing problems at reboot.

Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit d8cbfa101cbe3a9ece41120af93884177aff728a)