V4L/DVB (7379): tuner: prevent instance sharing if i2c adapter is NULL

We currently do not have a method to enable instance staring if i2c adapter
is NULL, in the cases of dvb demods that write to the tuner directly using
calc_regs.  Prevent possible wrong instance sharing for these cases until
a better solution can be found.

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