dmaengine: mpc512x: Fix fall-through warning for Clang

Fix the following fallthrough warning (powerpc-randconfig):

drivers/dma/mpc512x_dma.c:816:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

drivers/dma/mpc512x_dma.c

···
 	case 16:
 		if (is_mpc8308)
 			return false;
+		break;
 	case 1:
 	case 2:
 	case 4: