net/af_iucv: mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

net/iucv/af_iucv.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 537:3, 519:6, 2246:6, 510:6

Notice that, in this particular case, the code comment is
modified in accordance with what GCC is expecting to find.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>