isdn_v110: 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_v110.c: In function ‘EncodeMatrix’:
drivers/isdn/i4l/isdn_v110.c:353:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (line >= mlen) {
       ^
drivers/isdn/i4l/isdn_v110.c:358:3: note: here
   case 128:
   ^~~~

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>