net/mlx5: Fix error handling in load one

We didn't store the result of mlx5_init_once, due to that
mlx5_load_one returned success on error.  Fix that.

Fixes: 59211bd3

 ("net/mlx5: Split the load/unload flow into hardware and software flows")
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>