PCI: Remove NULL device handling from PCI DMA API

Historically some ISA drivers used the old PCI DMA API with a NULL pdev
argument, but these days this isn't used and not too useful due to the
per-device DMA ops, so remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>