mtd: create_freezable_workqueue() doesn't return an ERR_PTR

The create_freezable_workqueue() returns a NULL on error, it doesn't
return an ERR_PTR.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>