dma-mapping: remove dma_max_pfn

These days, the DMA mapping code must bounce buffers for any unsupported
address. If the driver needs to optimize for natively supported ranges,
then it should use dma_get_required_mask.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>