V4L/DVB (7102): make tuner-core.c:tuner_list static

tuner_list can become static - and it's anyway a way too generic name
for a global variable - see commit b00ef4b8d8c29bfb5f6f92ee60bc04b604f36ef2
for a completely different global variable of the same name I just made
static...

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>