V4L/DVB (11390): 2-dev.c: return 0 for NULL open and release callbacks

Patch allows v4l2_open and v4l2_release functions return 0 if open and
release driver callbacks set to NULL. This will be used in radio
drivers.

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Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>