V4L/DVB (5097): Convert cx8800 driver to video_ioctl2 handler

video_ioctl2 handler provides V4L2 API parsing.
Using it makes the driver simpler, and isolates API parsing.
This allows future reusage of driver controls using other ways, like sysfs
and/or procfs and increases isolation of driver-specific handling from the
generic common ioctl processing.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>