powerpc/dma: remove max_direct_dma_addr

The max_direct_dma_addr duplicates the bus_dma_mask field in struct
device.  Use the generic field instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>