[media] media: bt8xx: add missing put_device call

This is required so that we give up the last reference to the device.
Remove the kfree() because the put_device() call will actually call
release_sub_device which in turn kfrees the device.

Signed-off-by: Levente Kurusa <levex@linux.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>