parisc: ->mapping_error

DMA_ERROR_CODE already went away in linux-next, but parisc unfortunately
added a new instance of it without any review as far as I can tell.

Move the two iommu drivers to report errors through ->mapping_error.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Helge Deller <deller@gmx.de>