UBI: fix return error code

We are checking dfs_rootdir for error value or NULL. But in the
conditional ternary operator we returned -ENODEV if dfs_rootdir contains
an error value and returned PTR_ERR(dfs_rootdir) if dfs_rootdir is NULL.
So in the case of dfs_rootdir being NULL we actually assigned 0 to err
and returned it to the caller implying a success.
Lets return -ENODEV when dfs_rootdir is NULL else return
PTR_ERR(dfs_rootdir).

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>