V4L/DVB (7943): tuner: add macro, hybrid_tuner_report_instance_count

Create a macro to report the number of instances of the tuner driver
currently in use.  This will allow drivers to perform specific cleanups
before destroying the last instance of a tuner.

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