V4L/DVB (7137): tuner: return number of instances remaining after hybrid_tuner_release_state

Assign the number of instances remaining as the return value of
hybrid_tuner_release_state, in case there is any extra cleanup that
the tuner driver needs to do after an instance has been destroyed.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>