[media] em28xx: don't store usb_device at struct em28xx

Now that we're storing usb_interface at em28xx struct,
there's no good reason to keep storing usb_device, as we can
get it from usb_interface. So, get rid of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>