ser_gigaset: 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 warning:

drivers/isdn/gigaset/ser-gigaset.c: In function ‘gigaset_tty_ioctl’:
drivers/isdn/gigaset/ser-gigaset.c:627:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (arg) {
   ^~~~~~
drivers/isdn/gigaset/ser-gigaset.c:638:2: note: here
  default:
  ^~~~~~~

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>
Acked-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>