media: vidtv: add an initial channel frequency

Use 474 MHz frequency for DVB-T/DVB-C, as this is the at the
channel range that it is valid on most places for DVB-T.

In the case of DVB-S, let's add Astra 19.2E initial
frequency at the scan files as the default, e. g. 12.5515 GHz.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>