[media] si2168: Set symbol rate for DVB-C

This patch adds symbol rate setting to the driver.

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