V4L/DVB (11993): V4L/pwc - use usb_interface as parent, not usb_device

The current code creates a sysfs device path where the video4linux
device is child of the usb device itself instead of the interface it
belongs to. That is evil and confuses udev.

This patch does basically the same thing as Kay's similar patch for the
ov511 driver:

at git commit ce96d0a4

Signed-off-by: Lennart Poettering <mzxreary@0pointer.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>