powerpc/powernv: fix debugfs_create_dir() error checking

The debugfs_create_dir returns ERR_PTR incase of an error and the
correct way of checking it by using the IS_ERR inline function, and
not the simple null comparision. This patch fixes this.

Suggested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Signed-off-by: Immad Mir <mirimmad17@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/CY5PR12MB64553EE96EBB3927311DB598C6459@CY5PR12MB6455.namprd12.prod.outlook.com