media: i2c: imx214: Explain some magic numbers

Code refinement, no functional changes.

Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: André Apitzsch <git@apitzsch.eu>
[Sakari Ailus: The control handler remains in the context struct still.]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>