media: i2c: imx219: Replace exposure magic value with named constant

media: i2c: imx219: Replace exposure magic value with named constant

Introduce IMX219_EXPOSURE_OFFSET (4) and use it instead of the literal
'4' when computing the maximum coarse exposure. The IMX219 datasheet
specifies the maximum storage time as frame_length_lines - 4.
(Ref: Datasheet section 5-7-1)

Also fix one indentation issue for consistency.

Signed-off-by: Tarang Raval <tarang.raval@siliconsignals.io>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>