powerpc: expect all devices calling dma ops to have archdata set

Now that we set archdata for of_platform and platform devices via
platform_notify() we no longer need to special case having a NULL device
pointer or NULL archdata.  It should be a driver error if this condition
shows up and the driver should be fixed.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>