[POWERPC] Simplify dma_ops bug conditions

Use BUG_ON rather than BUG to simplify the dma_ops handing,
and remove the now-unnecessary return cases.

Booted on pseries.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>