mlx5: Restore err assignment in mlx5_mdev_init

Clang warns:

drivers/net/ethernet/mellanox/mlx5/core/main.c:1278:6: warning: variable
'err' is used uninitialized whenever 'if' condition is true
[-Wsometimes-uninitialized]
        if (!priv->dbg_root) {
            ^~~~~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlx5/core/main.c:1303:9: note:
uninitialized use occurs here
        return err;
               ^~~
drivers/net/ethernet/mellanox/mlx5/core/main.c:1278:2: note: remove the
'if' if its condition is always false
        if (!priv->dbg_root) {
        ^~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlx5/core/main.c:1259:9: note: initialize
the variable 'err' to silence this warning
        int err;
               ^
                = 0
1 warning generated.

This path previously returned -ENOMEM, restore that error code so that
it is not uninitialized.

Fixes: 810cbb2 ("net/mlx5: Add missing mutex destroy")
Link: ClangBuiltLinux#1042
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

drivers/net/ethernet/mellanox/mlx5/core/main.c

@@ -1277,6 +1277,7 @@ static int mlx5_mdev_init(struct mlx5_core_dev *dev, int profile_idx)
 					    mlx5_debugfs_root);
 	if (!priv->dbg_root) {
 		dev_err(dev->device, "mlx5_core: error, Cannot create debugfs dir, aborting\n");
+		err = -ENOMEM;
 		goto err_dbg_root;
 	}