UBI: fix debugfs-less systems support

Commit "aa44d1d3 UBI: remove Kconfig debugging option" broke UBI and it
refuses to initialize if debugfs (CONFIG_DEBUG_FS) is disabled. I incorrectly
assumed that debugfs files creation function will return success if debugfs
is disabled, but they actually return -ENODEV. This patch fixes the issue.
Reported-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Tested-by: Paul Parsons <lost.distance@yahoo.com>

drivers/mtd/ubi/debug.c

@@ -264,6 +264,9 @@ static struct dentry *dfs_rootdir;
  */
 int ubi_debugfs_init(void)
 {
+	if (!IS_ENABLED(DEBUG_FS))
+		return 0;
+
 	dfs_rootdir = debugfs_create_dir("ubi", NULL);
 	if (IS_ERR_OR_NULL(dfs_rootdir)) {
 		int err = dfs_rootdir ? -ENODEV : PTR_ERR(dfs_rootdir);
@@ -281,6 +284,7 @@ int ubi_debugfs_init(void)
  */
 void ubi_debugfs_exit(void)
 {
+	if (IS_ENABLED(DEBUG_FS))
 		debugfs_remove(dfs_rootdir);
 }
 
@@ -403,6 +407,9 @@ int ubi_debugfs_init_dev(struct ubi_device *ubi)
 	struct dentry *dent;
 	struct ubi_debug_info *d = ubi->dbg;
 
+	if (!IS_ENABLED(DEBUG_FS))
+		return 0;
+
 	n = snprintf(d->dfs_dir_name, UBI_DFS_DIR_LEN + 1, UBI_DFS_DIR_NAME,
 		     ubi->ubi_num);
 	if (n == UBI_DFS_DIR_LEN) {
@@ -470,5 +477,6 @@ int ubi_debugfs_init_dev(struct ubi_device *ubi)
  */
 void ubi_debugfs_exit_dev(struct ubi_device *ubi)
 {
+	if (IS_ENABLED(DEBUG_FS))
 		debugfs_remove_recursive(ubi->dbg->dfs_dir);
 }