[media] tda665x: split set_frequency from set_state

On tda665x, set_state only sets frequency. As the kABI for set_state
is meant to be used only on special cases, split the function
into two, in order to allow it to be latter used by a DVBv5
cache params logic.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>