V4L/DVB (4872): Saa7134-alsa improvements

The change does the following:
- At device open, it sets the recording source to the current
  input instead of LINE2. So it is no longer necessary to set the
  recording source with a mixer application.
- Connects the mixer volume control to the input sensitivity selection
  of the analog sound inputs. This allows only one 6db step.

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>