mtd: nand: nandsim: fix error check

debugfs_create_dir() and debugfs_create_file() returns NULL on error or
a pointer on success. They do not return the error value with ERR_PTR.
So we should not check the return with IS_ERR_OR_NULL, instead we
should just check for NULL.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>