media: ov2640: set default window and format code at probe time

Set default window and format code at probe time instead of always checking
if they have not been set yet when VIDIOC_SUBDEV_G_FMT ioctl is called.

This change simplifies the next patch (make VIDIOC_SUBDEV_G_FMT ioctl work
with V4L2_SUBDEV_FORMAT_TRY).

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>