media: i2c: add ov7251_init_ctrls()

V4L2 controls initialisation takes up a sizeable portion of the
driver's .probe() function. To keep things neat, move it to a
dedicated function.

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>