[media] em28xx: Only deallocate struct em28xx after finishing all extensions

We can't free struct em28xx while one of the extensions is still
using it.

So, add a kref() to control it, freeing it only after the
extensions fini calls.

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