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 <gregkh@linuxfoundation.org>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>