media: staging: media: zoran: move videodev alloc

Move some code out of zr36057_init() and create new functions for handling
zr->video_dev. This permit to ease code reading and fix a zr->video_dev
memory leak.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>