V4L/DVB (5644): Usbvision: video_ioctl2 conversion

The ioctl entry point, a big switch/case, is splitted in little 
functions. 

These functions are set as callbacks for the video_ioctl2 video4linux 
facility.

This improves the driver memory consumption and enables the v4l1 
compatibility as a side effect.

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Acked-by: Dwaine P. Garden <dwainegarden@rogers.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>