Don't mix NULL and ERR_PTR returns.

Fixes: 2e87570b ("nvme-pci: factor out a nvme_pci_alloc_dev helper")
Signed-off-by: Irvin Cote <irvin.cote@insa-lyon.fr>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>