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 <miltonm@bga.com>
Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>