media: i2c: Extend .get_selection() for ov7251

Extend the .get_selection() callback to support other values for
sel->target, primarily to satisfy libcamera's requirements.

Signed-off-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>