V4L/DVB (4959): Usbvision: possible cleanups

This patch contains the following possible cleanups:
- make needlessly global functions static
- remove the unused EXPORT_SYMBOL's

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>