[media] au0828: Move the Kconfig knob under V4L_USB_DRIVERS

This driver is for USB devices, but was incorrectly listed under
V4L_PCI_DRIVERS.

Signed-off-by: Ismael Luceno <ismael.luceno@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>