V4L/DVB: pwc: fully convert driver to V4L2

Remove the V4L1 API from this driver, making it fully V4L2.

Also fix a bug where the /dev/videoX device was created too early, which led
to initialization problems of the camera, making it unable to capture video.

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