mmc: exynos_dw_mmc: Fix getting private data in exynos_dwmci_board_init()

mmc: exynos_dw_mmc: Fix getting private data in exynos_dwmci_board_init()

In case of CONFIG_DM_MMC, host->priv actually holds (struct udevice *),
and not (struct dwmci_exynos_priv_data *). This makes *priv pointer
invalid and may lead to Synchronous Abort during its dereference later
in exynos_dwmci_board_init(). Fix it by extracting
exynos_dwmmc_get_priv() helper from exynos_dwmci_clksel() and using it
for getting the private data in exynos_dwmci_board_init()

Fixes: 3537ee879e04 ("mmc: exynos_dw_mmc: support the Driver mode for Exynos")
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>