m68k: Set default dma mask for platform devices

This avoids a WARNING splat when loading the macsonic or macmace driver.
Please see commit 205e1b7f ("dma-mapping: warn when there is no
coherent_dma_mask").

This implementation of arch_setup_pdev_archdata() differs from the
powerpc one, in that this one avoids clobbering a device dma mask
which has already been initialized.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>