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

The HBLANK control was read-only, and always configured such
that the sensor HTS 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 HTS setting when changing modes rather than
resetting it to a default.

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