firewire: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/firewire/core-device.c: In function ‘set_broadcast_channel’:
drivers/firewire/core-device.c:969:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (data & cpu_to_be32(1 << 31)) {
       ^
drivers/firewire/core-device.c:974:3: note: here
   case RCODE_ADDRESS_ERROR:
   ^~~~
drivers/firewire/core-iso.c: In function ‘manage_channel’:
drivers/firewire/core-iso.c:308:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if ((data[0] & bit) == (data[1] & bit))
       ^
drivers/firewire/core-iso.c:312:3: note: here
   default:
   ^~~~~~~
drivers/firewire/core-topology.c: In function ‘count_ports’:
drivers/firewire/core-topology.c:69:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
    (*child_port_count)++;
    ~~~~~~~~~~~~~~~~~~~^~
drivers/firewire/core-topology.c:70:3: note: here
   case SELFID_PORT_PARENT:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that in some cases, the code comment is modified in
accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Cc: Kees Cook <keescook@chromium.org>
Cc: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (reworded a comment)
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/firewire/core-device.c

@@ -957,7 +957,7 @@ static void set_broadcast_channel(struct fw_device *device, int generation)
 				device->bc_implemented = BC_IMPLEMENTED;
 				break;
 			}
-			/* else fall through to case address error */
+			/* else, fall through - to case address error */
 		case RCODE_ADDRESS_ERROR:
 			device->bc_implemented = BC_UNIMPLEMENTED;
 		}

drivers/firewire/core-iso.c

@@ -284,7 +284,7 @@ static int manage_channel(struct fw_card *card, int irm_id, int generation,
 			if ((data[0] & bit) == (data[1] & bit))
 				continue;
 
-			/* 1394-1995 IRM, fall through to retry. */
+			/* fall through - It's a 1394-1995 IRM, retry. */
 		default:
 			if (retry) {
 				retry--;

drivers/firewire/core-topology.c

@@ -54,6 +54,7 @@ static u32 *count_ports(u32 *sid, int *total_port_count, int *child_port_count)
 		switch (port_type) {
 		case SELFID_PORT_CHILD:
 			(*child_port_count)++;
+			/* fall through */
 		case SELFID_PORT_PARENT:
 		case SELFID_PORT_NCONN:
 			(*total_port_count)++;