[media] rtl2832: properly set en_bbin for r820t

DVBT_EN_BBIN should be set on both places where IF is set. So,
move it to a function and call it where needed.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Antti Palosaari <crope@iki.fi>