powerpc: remove support for NULL dev in __phys_to_dma / __dma_to_phys

Support for calling the DMA API functions without a valid device pointer
was removed a while ago, so remove the stale support for that from the
powerpc __phys_to_dma / __dma_to_phys helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>