V4L/DVB (5039): Pvrusb2: Implement /dev/radioX

The "main" V4L2 interface patch. This is yet very incomplete, incorrect and
probably inappropriate for inclusion as-is, but at least with this I 'm able
to tune and play radio through a V4L2 program (pvr-radio.c, a "thumb" version
of ivtv-radio.c with just the essentials).

Therefore, it kinda gives an idea of what is needed to support this, hm,
interface (partly used also by e.g., kradio). Please point out any mistakes
on this code. I 'm sure I 'm messing up some struct initialization somewhere
but currently I 'm too lazy to actually think this through until I complete
the functionality (e.g., handle the VIDIOC_S_STD, ENUMINPUT, etc ioctls
appropriately).

Signed-off-by: Pantelis Koukousoulas <pakt223@freemail.gr>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>