media: i2c: ov9281: Increase diff between VTS and max exposure

The driver did allow the exposure to go up to VTS - 4 lines,
but this would produce a visible line on 1280x800, and a stall of
the sensor at 640x480.

Whilst it appears to work with a difference of 5, the datasheet states
there should be at least 25 lines difference between VTS and exposure,
so use that value.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>