[media] si2165: Prepare si2165_set_frontend() for future DVB-C support

The current si2165_set_frontend() function actually works only for DVB-T.
Split it into two functions, preparing the code to support DVB-C.

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