[media] dib8000: fix compiler warning

In file included from include/uapi/linux/posix_types.h:4:0,
                 from include/uapi/linux/types.h:13,
                 from include/linux/types.h:5,
                 from include/uapi/linux/sysinfo.h:4,
                 from include/uapi/linux/kernel.h:4,
                 from include/linux/cache.h:4,
                 from include/linux/time.h:4,
                 from include/linux/input.h:11,
                 from drivers/media/usb/dvb-usb/dvb-usb.h:13,
                 from drivers/media/usb/dvb-usb/dib0700.h:13,
                 from drivers/media/usb/dvb-usb/dib0700_devices.c:9:
drivers/media/dvb-frontends/dib8000.h: In function 'dib8000_attach':
include/linux/stddef.h:8:14: warning: return makes integer from pointer without a cast [-Wint-conversion]
 #define NULL ((void *)0)
              ^
drivers/media/dvb-frontends/dib8000.h:72:9: note: in expansion of macro 'NULL'
  return NULL;
         ^

Reported-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>