[media] pwc: Move various initialization to driver load and / or stream start

Doing a bunch of initialization every time /dev/video is opened, and thus
for example when the udev rules probe for capabilities makes no sense,
do it at driver load, resp. stream start instead.

This is a preparation patch for allowing multiple opens of the /dev/video
node.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>