Fix the following fallthrough warning:

arch/powerpc/platforms/pasemi/idle.c:45:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60efbf18.d9n6eXv275OJcc7T%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>