hw/isa: Rename isa_get_dma() -> isa_bus_get_dma()

hw/isa: Rename isa_get_dma() -> isa_bus_get_dma()

isa_get_dma() returns a DMA channel handler from an ISABus.
To emphasize this, rename it as isa_bus_get_dma().

Mechanical change using:

  $ sed -i -e 's/isa_get_dma/isa_bus_get_dma/g' \
        $(git grep -l isa_get_dma)

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230215161641.32663-2-philmd@linaro.org>