soc: mediatek: mtk-svs: Use ULONG_MAX to compare floor frequency

The `freq` variable is of type unsigned long and, even though it does
currently work with u32 because no frequency is higher than U32_MAX,
it is not guaranteed that in the future we will see one.
Initialize the freq variable with ULONG_MAX instead of U32_MAX.

Link: https://lore.kernel.org/r/20231121125044.78642-18-angelogioacchino.delregno@collabora.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>