V4L/DVB (4406): Convert radio-cadet to V4L2 API

This is a card with RDS capabilities. 
RDS specifications didn't change from V4L1 to V4L2, so that part should be OK. 
This patch changed the following stuff:
* The device can be opened multiple times. That's necessary because there are 
at least a radio application and an RDS application (rdsd) that want to 
open() the device.
* Added a poll() function. Every character device should have that, and rdsd 
expects it as it uses select() on that file descriptor.
* Converted the ioctls to V4L2. MUTE is not implemented correctly as the 
card doesn't seem to have a special bit for that. Probably there are a few 
more ioctls that should at least return 0 or an error.
As I do not own such a card, I couldn't test anything. If there is anybody out 
there who owns such an ancient card, please test and report. 
I just checked that the code compiles.

Signed-off-by: Hans J. Koch <koch@hjk-az.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>