media: ov2740: change the minimal exposure value to 4

The minimal valid exposure value should be 4 lines instead of 8 for
ov2740 mannual exposure control.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>