media: ov5675: correct the maximum exposure value

The unit of exposure value is different from other OmniVision sensors,
driver will divide by 2 before set register, the exposure range exposed
by v4l2 ctrl to user should be same as others, so the calculation for
the maximum exposure value in current driver need be fixed.

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>