media: vidtv: fix initialization of the network_id field at SDT

This field should point to the network ID, and has different
ranges for cable, terrestrial or satellite. It also has
an special range for temporary private usage.

For now, let's use the temporary private one. Once the
Network Information Table (NIT) gets added, this should be
reviewed.

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