V4L/DVB (4937): Usbvision cleanup and code reorganization

- removal of overlay stuff
- reorganization of functions in 3 files:
    * usbvision-i2c for I2C-related stuff
    * usbvision-video for v4l2 entry points
    * usbvision-core for all peripheral controls and utilities

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