docs: firmware-guide: ACPI: Replace dma_request_slave_channel() with dma_request_chan()

dma_request_chan() is the preferred API to request slave channels.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>