media: i2c: ov5647: Correct pixel array offset

The top offset in the pixel array is actually 6 (see page 3-1 of the
OV5647 data sheet).

Fixes: f2f7ad5c ("media: i2c: ov5647: Selection compliance fixes")
Signed-off-by: David Plowman <david.plowman@raspberrypi.com>