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 <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60ef0750.I8J+C6KAtb0xVOAa%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/dma/mpc512x_dma.c

@@ -813,6 +813,7 @@ inline bool is_buswidth_valid(u8 buswidth, bool is_mpc8308)
 	case 16:
 		if (is_mpc8308)
 			return false;
+		break;
 	case 1:
 	case 2:
 	case 4: