media: i2c: rdacm21: Fix uninitialized value

Fix the following smatch warning:

drivers/media/i2c/rdacm21.c:373 ov10640_check_id() error: uninitialized
symbol 'val'.

Initialize 'val' to 0 in the ov10640_check_id() function.

Fixes: 2b821698 ("media: i2c: rdacm21: Power up OV10640 before OV490")
Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>