nubus: Set default dma mask for nubus_board devices

A 32-bit mask is used by default because a NuBus slot has 32
address/data lines and a NuBus board is free to use all of them.

Cc: Greg Kroah-Hartman <[e-mail]>
Tested-by: Stan Johnson <[e-mail]>
Signed-off-by: Finn Thain <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Geert Uytterhoeven <[e-mail]>

drivers/nubus/bus.c

···
 // Copyright (C) 2017 Finn Thain
 
 #include <linux/device.h>
+#include <linux/dma-mapping.h>
 #include <linux/list.h>
 #include <linux/nubus.h>
 #include <linux/seq_file.h>
···
 	board->dev.release = nubus_device_release;
 	board->dev.bus = &nubus_bus_type;
 	dev_set_name(&board->dev, "slot.%X", board->slot);
+	board->dev.dma_mask = &board->dev.coherent_dma_mask;
+	dma_set_mask(&board->dev, DMA_BIT_MASK(32));
 	return device_register(&board->dev);
 }