media: i2c: ov5647: Correct minimum VBLANK value

Trial and error reveals that the minimum vblank value appears to be 24
(the OV5647 data sheet does not give any clues). This fixes streaming
lock-ups in full resolution mode.

Fixes: 9b5a5ebe ("media: i2c: ov5647: Add support for V4L2_CID_VBLANK")
Signed-off-by: David Plowman <david.plowman@raspberrypi.com>