[media] m88ds3103: possible uninitialized scalar variable

It was possible that tuner_frequency variable, used for carrier offset
compensation, was uninitialized. That happens when tuner
.get_frequency() callback is not defined.

Currently that case is not possible as only used tuner has this callback.

Coverity CID 1166057: Uninitialized scalar variable (UNINIT)

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