soc: mtk-svs: mt8183: refactor o_slope calculation

The o_slope value is dependent of the o_slope_sign, refactor code to get
rid of unnecessary if constructs.

Signed-off-by: Roger Lu <roger.lu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230111074528.29354-15-roger.lu@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>