media: atomisp: fix enum formats logic

Changeset 374d62e7 ("media: v4l2-subdev: Verify v4l2_subdev_call() pad config argument")
added an extra verification for a pads parameter for enum mbus
format code.

Such change broke atomisp, because now the V4L2 core
refuses to enum MBUS formats if the state is empty.

So, add .which field in order to select the active formats,
in order to make it work again.

While here, improve error messages.

Fixes: 374d62e7 ("media: v4l2-subdev: Verify v4l2_subdev_call() pad config argument")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>