V4L/DVB (7995): si470x: a lot of small code cleanups

This patch brings the following changes:
- comment on how to listen to an usb audio device
  (i get so many questions about that...)
- code cleanup (error handling, more warnings, spacing, ...)

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>