media: atomisp-ov2680: properly set the vts value

The vts value should be set before being checked, as otherwise a
warning will arise:

	drivers/staging/media/atomisp/i2c/atomisp-ov2680.c: In function 'ov2680_set_fmt':
	drivers/staging/media/atomisp/i2c/atomisp-ov2680.c:873:33: warning: 'vts' may be used uninitialized
	[-Wmaybe-uninitialized]
	  873 |         if (dev->exposure > vts - OV2680_INTEGRATION_TIME_MARGIN)

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Fixes: 62b984359b6f ("media: atomisp-ov2680: Fix ov2680_set_fmt() messing up high exposure settings")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>