As such macro will check if the expression is true, it may fall through, as
warned:

In file included from ./include/uapi/linux/stddef.h:1:0,
                 from ./include/linux/stddef.h:4,
                 from ./include/uapi/linux/posix_types.h:4,
                 from ./include/uapi/linux/types.h:13,
                 from ./include/linux/types.h:5,
                 from ./drivers/media/dvb-core/dvb_frontend.h:35,
                 from drivers/media/tuners/fc0011.h:4,
                 from drivers/media/tuners/fc0011.c:20:
drivers/media/tuners/fc0011.c: In function 'fc0011_set_params':
./include/linux/compiler.h:179:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:109:2: note: in expansion of macro 'unlikely'
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
drivers/media/tuners/fc0011.c:344:3: note: in expansion of macro 'WARN_ON'
   WARN_ON(1);
   ^~~~~~~
drivers/media/tuners/fc0011.c:345:2: note: here
  case 0:
  ^~~~
In file included from ./include/uapi/linux/stddef.h:1:0,
                 from ./include/linux/stddef.h:4,
                 from ./include/uapi/linux/posix_types.h:4,
                 from ./include/uapi/linux/types.h:13,
                 from ./include/linux/types.h:5,
                 from ./include/linux/list.h:4,
                 from ./include/linux/module.h:9,
                 from drivers/media/pci/tw5864/tw5864-video.c:17:
drivers/media/pci/tw5864/tw5864-video.c: In function 'tw5864_fmt_vid_cap':
./include/linux/compiler.h:179:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
./include/asm-generic/bug.h:68:2: note: in expansion of macro 'unlikely'
  unlikely(__ret_warn_on);    \
  ^~~~~~~~
drivers/media/pci/tw5864/tw5864-video.c:547:3: note: in expansion of macro 'WARN_ON_ONCE'
   WARN_ON_ONCE(1);
   ^~~~~~~~~~~~
drivers/media/pci/tw5864/tw5864-video.c:548:2: note: here
  case STD_NTSC:
  ^~~~

On both cases, it means an error, so, let's return an error
code, to make gcc happy.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Michael Büsch <m@bues.ch>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>