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>

arch/powerpc/platforms/powermac/smp.c

@@ -146,6 +146,7 @@ static inline void psurge_clr_ipi(int cpu)
 		switch(psurge_type) {
 		case PSURGE_DUAL:
 			out_8(psurge_sec_intr, ~0);
+			break;
 		case PSURGE_NONE:
 			break;
 		default: