media: i2c: ds90ub960: Fix uninitialized variable in ub960_serializer_temp_ramp()

The "ret" variable is not initialized on the success path.

Fixes: a0574474 ("media: i2c: ds90ub9xx: Set serializer temperature ramp")
Cc: stable@vger.kernel.org
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>