V4L/DVB (6878): tuner: remove extraneous variable declaration

tuner_count is already declared as "extern unsigned const int"
in <media/tuner-types.h>  -- Remove it from tuner-driver.h

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