[media] si2168: add ts_mode setting and move to si2168_init

Luis Alves submitted a TS mode patch to si2168 earlier, but the
patch was rejected due to a small issue. Here is a working version.
Also, setting of TS mode is moved from si2168_set_frontend to
si2168_init.

This patch adds the TS mode as a config option for the si2168 demod:
- ts_mode added to config struct.
- Possible (interesting) values are
   * Parallel mode = 0x06
   * Serial mode = 0x03

Currently the modules using this demod only use parallel mode.
Patches for these modules later in this patch series.

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