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>