[media] usbvision: move init code to probe()

These things are only initialized if you start streaming video, but
they are also used in the disconnect function. So just init them
always during probe time.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>