isdn_common: 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/isdn/i4l/isdn_common.c: In function ‘isdn_wildmat’:
drivers/isdn/i4l/isdn_common.c:173:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
    p++;
    ~^~
drivers/isdn/i4l/isdn_common.c:174:3: note: here
   default:
   ^~~~~~~
  CC [M]  drivers/leds/leds-lp8788.o
  CC [M]  drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/smu10_smumgr.o
drivers/isdn/i4l/isdn_common.c: In function ‘isdn_status_callback’:
drivers/isdn/i4l/isdn_common.c:729:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (divert_if)
      ^
drivers/isdn/i4l/isdn_common.c:732:2: 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>
Signed-off-by: David S. Miller <davem@davemloft.net>