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>

Documentation/DMA-mapping.txt

@@ -317,9 +317,9 @@ you should do:
 
 	cpu_addr = pci_alloc_consistent(pdev, size, &dma_handle);
 
-where pdev is a struct pci_dev *. You should pass NULL for PCI like buses
-where devices don't have struct pci_dev (like ISA, EISA).  This may be
-called in interrupt context. 
+where pdev is a struct pci_dev *. This may be called in interrupt context.
+You should use dma_alloc_coherent (see DMA-API.txt) for buses
+where devices don't have struct pci_dev (like ISA, EISA).
 
 This argument is needed because the DMA translations may be bus
 specific (and often is private to the bus which the device is attached