isdn: i4l: isdn_tty: Mark expected switch fall-through

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

This patch fixes the following warnings:

drivers/isdn/i4l/isdn_tty.c: In function ‘isdn_tty_edit_at’:
drivers/isdn/i4l/isdn_tty.c:3644:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
       m->mdmcmdl = 0;
       ~~~~~~~~~~~^~~
drivers/isdn/i4l/isdn_tty.c:3646:5: note: here
     case 0:
     ^~~~

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

Notice that, in this particular case, 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.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>