PCI: Fix fall-through warning for Clang

Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/pci/proc.c:234:3: warning: fallthrough annotation in unreachable code [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/pci/proc.c

@@ -230,8 +230,8 @@ static long proc_bus_pci_ioctl(struct file *file, unsigned int cmd,
 			break;
 		}
 		/* If arch decided it can't, fall through... */
-#endif /* HAVE_PCI_MMAP */
 		fallthrough;
+#endif /* HAVE_PCI_MMAP */
 	default:
 		ret = -EINVAL;
 		break;