powerpc/smp: Fix fall-through warning for Clang

Fix the following fallthrough warning:

arch/powerpc/platforms/powermac/smp.c:149:3: 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>