V4L/DVB (6088): cx2341x: some controls can't be changed while the device is busy

The driver should now pass the 'busy' state of the device to the cx2341x 
module whenever controls are set or tried. -EBUSY will be returned if 
the device is busy and the user attempts to modify certain 'dangerous' 
controls. It concerns controls that change the audio or video 
compression mode and bitrates.

The cx88-blackbird and pvrusb2 drivers currently always pass '0' (not busy)
to the cx2341x, effectively keeping the old behavior for now.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>