mmc: exynos_dw_mmc: set to clksel_val into board-init function

"clksel_val" is assigned to property of mmc or defined value.
But it doesn't write at initial sequence.
There is a reason that get the wrong source-clock value.
This patch fixed it.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>