[media] cx24120: declare cx24120_init() as static

drivers/media/dvb-frontends/cx24120.c:1182:5: warning: no previous prototype for 'cx24120_init' [-Wmissing-prototypes]
 int cx24120_init(struct dvb_frontend *fe)
     ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>