media: ov2680: Fix ov2680_enum_frame_interval()

Fix and simplify ov2680_enum_frame_interval(), the index is not
an index into ov2680_mode_data[], so using OV2680_MODE_MAX is wrong.

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

Note validating fie->which is not necessary this is already done
by the v4l2-core.

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>