media: atomisp: ov2680: Fix ov2680_enum_frame_interval()

Fix and simplify ov2680_enum_frame_interval(), the index is not
an index into ov2680_res_preview[], so using N_PREVIEW is wrong.

Instead it is an index indexing the different framerates for
the resolution specified in fie->width, fie->height.

Since the ov2680 code only supports a single fixed 30 fps,
index must always be 0 and we don't need to check the other
fie input values.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>