media: i2c: imx219: make HBLANK r/w to allow longer exposures

The HBLANK control was read-only, and always configured such that the
sensor line length register was 3448. This limited the maximum exposure
time that could be achieved to around 1.26 secs.

Make HBLANK read/write so that the line time can be extended, and
thereby allow longer exposures (and slower frame rates). Retain the
overall line length setting when changing modes rather than resetting it
to a default.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Jai Luthra <jai.luthra@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>