media: mt9m114: Lower minimum vblank value

media: mt9m114: Lower minimum vblank value

As the comment above the defines says, the minimum values are undocumented
so the lowest values seen in register lists are used.

The version of the mt9m114 driver shipped together with the atomisp code
uses 21 for vblank in its register lists, lower MT9M114_MIN_VBLANK
accordingly.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>