V4L/DVB (3709): Improve line-in handling

- improve handling of the EXTERN input: don't start an unnecessary carrier scan
- improve the LOG_STATUS output
- ensure that a carrier scan is started again when switching back to the tuner.
- set correct prescale for L-NICAM

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>