[media] dib8000: export just one symbol

Exporting multiple symbols don't work as it causes compilation
breakages, due to the way dvb_attach() works.

The bug happens when:
        CONFIG_DVB_DIB8000=m
	CONFIG_DVB_USB_DIB0700=y

As a bonus, dib8000 won't be loaded anymore if the device uses
a different frontend, reducing the memory footprint.

Tested with both Pixelview PV-D231 and MyGica S870.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>