media: rockchip: rkisp1: reduce number of histogram grid elements in uapi

The uapi right now specifies an array size of 28 but the actual number
of elements is only 25 with the last 3 being unused.

Reduce the array size to the correct number of elements and change
the params code to iterate the array 25 times.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>