media: atomisp: Fix error code in ov5693_probe()

If gmin_camera_platform_data() returns NULL then we should return a
negative error instead of success.

Fixes: 90ebe55a

 ("media: staging: atomisp: Add driver prefix to Kconfig option and module names")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>