powerpc/dma: Fix check for direct DMA support

The current check is wrong because it does not take the DMA offset intot
account, and in the case of a driver which doesn't actually support
64bits would falsely report that device as working.

Signed-off-by: Milton Miller <[e-mail]>
Signed-off-by: Nishanth Aravamudan <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/kernel/dma.c

···
 	/* Could be improved so platforms can set the limit in case
 	 * they have limited DMA windows
 	 */
-	return mask >= (memblock_end_of_DRAM() - 1);
+	return mask >= get_dma_offset(dev) + (memblock_end_of_DRAM() - 1);
 #else
 	return 1;
 #endif