doc: fix an incorrect suggestion to pass NULL for PCI like buses

Fix an incorrect suggestion to pass NULL to pci_alloc_consistent
for PCI like buses where devices don't have struct pci_dev (like ISA, EISA).

Signed-off-by: Matti Linnanvuori <mattilinnanvuori@yahoo.com>
Acked-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@hobbes.lan>