media: i2c: ov9282: Make V4L2_CID_HBLANK r/w

There's no reason why HBLANK has to be read-only as it
only changes the TIMING_HTS register in the sensor.

Remove the READ_ONLY flag, and add the relevant handling
for it.

The minimum value also varies based on whether continuous clock
mode is being used or not, so allow hblank_min to depend on
that.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>