dma-mapping: better document dma_addr_t and DMA_MAPPING_ERROR

Move the comment documenting dma_addr_t away from the dma_map_ops
definition which isn't very related to it, and toward DMA_MAPPING_ERROR,
which is somewhat related.  Add a little blurb about DMA_MAPPING_ERROR
as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>