[media] cx231xx_dvb: use demod_i2c for demod attach

Tested:
* CX231XX_BOARD_HAUPPAUGE_930C_HD_1113xx
* CX231XX_BOARD_HAUPPAUGE_930C_HD_1114xx

Not Tested:
* CX231XX_BOARD_HAUPPAUGE_EXETER
* CX231XX_BOARD_HAUPPAUGE_955Q

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>