[media] mxl111sf: Fix compile when CONFIG_DVB_USB_MXL111SF is unset

Fix the following build error:

drivers/media/usb/dvb-usb-v2/
mxl111sf-tuner.h:72:9: error: expected ‘;’, ‘,’ or ‘)’ before ‘struct’
         struct mxl111sf_tuner_config *cfg)

Signed-off-by: Dave Jones <davej@fedoraproject.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>