media: i2c: ov7251: Add V4L2_CID_HBLANK control

HBLANK is a fixed value in this driver, so add as a fixed
read-only control.
The value is updated during set_format as the V4L2 control is
dependent on width, even though only one width is actually
defined.

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