media: atomisp: don't store an unused sink data on a var

Fixes this Werror breakage:

drivers/staging/media/atomisp/pci/atomisp_ioctl.c: In function 'atomisp_streamon':
drivers/staging/media/atomisp/pci/atomisp_ioctl.c:1714:44: error: variable 'sink' set but not used [-Werror=unused-but-set-variable]
 1714 |                 struct v4l2_mbus_framefmt *sink;
      |                                            ^~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>