Documentation/DMA-ISA-LPC: fix an incorrect reference

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To translate the virtual address to a bus address, use the normal DMA

API. Do _not_ use isa_virt_to_phys() even though it does the same

thing. The reason for this is that the function isa_virt_to_phys()

API. Do _not_ use isa_virt_to_bus() even though it does the same

thing. The reason for this is that the function isa_virt_to_bus()

will require a Kconfig dependency to ISA, not just ISA_DMA_API which

is really all you need. Remember that even though the DMA controller

has its origins in ISA it is used elsewhere.