media: atomisp-ov2680: initialize return var

As the settings are only applied when the device is powered on,
it should return 0 when the device is not powered.

Not doing that causes a warning:

	drivers/staging/media/atomisp/i2c/atomisp-ov2680.c: In function 'ov2680_ioctl':
	drivers/staging/media/atomisp/i2c/atomisp-ov2680.c:390:16: warning: 'ret' may be used uninitialized in this
	function [-Wmaybe-uninitialized]
	  390 |         return ov2680_set_exposure(sd, coarse_itg, analog_gain, digital_gain);
	      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	drivers/staging/media/atomisp/i2c/atomisp-ov2680.c:359:13: note: 'ret' was declared here
	  359 |         int ret;
	      |             ^~~
Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Fixes: 6b5b60687ada ("media: atomisp-ov2680: Save/restore exposure and gain over sensor power-down")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/staging/media/atomisp/i2c/atomisp-ov2680.c

@@ -356,7 +356,7 @@ static int ov2680_set_exposure(struct v4l2_subdev *sd, int exposure,
 			       int gain, int digitgain)
 {
 	struct ov2680_device *dev = to_ov2680_sensor(sd);
-	int ret;
+	int ret = 0;
 
 	mutex_lock(&dev->input_lock);