[media] pvrusb2: use struct v4l2_fh

By using struct v4l2_fh both the prio handling and the linked list
implementation in pvrusb2 can be removed since both are now done in
the v4l2 core if you use struct v4l2_fh.

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