V4L/DVB (7061): radio-si470x: Some cleanups

- code reordered to avoid function prototypes
- switch/case defaults are now more user-friendly
- unified comment style
- applied all checkpatch.pl v1.12 suggestions
  except the warning about the too long lines with bit comments
- renamed FMRADIO to RADIO to cut line length (checkpatch.pl)

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