i2c: mediatek: modify bus speed calculation formula

When clock-div is 0 or greater than 1, the bus speed
calculated by the old speed calculation formula will be
larger than the target speed. So we update the formula.

Signed-off-by: Kewei Xu <kewei.xu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>