powerpc/dma: remove unnecessary BUG()

Direction is already checked in all calling functions in
include/linux/dma-mapping.h and also in called function __dma_sync()

So really no need to check it once more here.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>