V4L/DVB (6460): v4l2-i2c-drv: add legacy_probe function pointer

Some devices do complicated tests whether the device can be probed or not.
Add a legacy_probe function pointer to support that.

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