media: si2157: Better check for running tuner in init

Getting the Xtal trim property to check if running is less error prone.
Reset if_frequency if state is unknown.

Replaces the previous "garbage check".

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>