[media] em28xx: always call em28xx_release_resources() in the usb disconnect handler

When the usb device is disconnected, the resources are no longer available,
so there is no reason to keep them registered.

This will also fix the various sysfs group removal warnings which we can see
since kernel 3.13.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>