[media] cxd2820r_c: fix if_ctl calculation

fixes tuning for DVB-C

Signed-off-by: Evgeny Plehov <EvgenyPlehov@ukr.net>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>