media/dvb: add __init/__exit macros to bt878.c

Trivial patch which adds the __init/__exit macros to the module_init/
module_exit functions of

drivers/media/dvb/bt8xx/bt878.c

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>