[media] rtl2832: remove unused if_dvbt config parameter

All used tuners has get_if_frequency() callback and that parameter
is not needed and will not needed as all upcoming tuner drivers
should implement get_if_frequency().

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>