net/mlx5: Continue driver initialization despite debugfs failure

The failure to create debugfs entry is unpleasant event, but not enough
to abort drier initialization. Align the mlx5_core code to debugfs design
and continue execution whenever debugfs_create_dir() successes or not.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>