[media] dvb/drxd: stub out drxd_attach when not built

This avoids getting
drivers/media/video/em28xx/em28xx-dvb.c:721: \
                       undefined reference to `drxd_attach'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>