arm: exynos: fix the div value for set_mmc_clk

The most exynos used the  "Ratio + 1" as div value.
And value at register is "Ratio".
So if want to set exact value, it needs to subtract one.

Value at register ("Ratio") = div - 1

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>