V4L/DVB (5206): Usbvision: set alternate interface modification

- usb alternate selection modified to get the biggest endpoint packet size.
- fix sysfs get values for brightness/contrast/hue/saturation

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>