sparc: set a default 32-bit dma mask for OF devices

This keeps the historic default behavior for devices without a DMA mask,
but removes the warning about a lacking DMA mask for doing DMA without
a mask.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>