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>