V4L/DVB (4885): Improve saa711x check

The old code would accept any device on the same i2c address as the
saa711x chips as an saa711x. However, this fails with saa717x chips,
which use that same address and so are misdetected as a saa7111. Now
check whether the chip is really a saa711x model.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>