m68k: Set default dma mask for platform devices

m68k: Set default dma mask for platform devices

This avoids a WARNING splat when loading the macsonic or macmace driver.
Please see commit 205e1b7f51e4 ("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 <[email protected]>
Cc: Greg Ungerer <[email protected]>
Signed-off-by: Finn Thain <[email protected]>
Acked-by: Greg Ungerer <[email protected]>
Signed-off-by: Geert Uytterhoeven <[email protected]>