mach-exynos: clock: restore calling dead exynos4_get_mmc_clk()

After rework of code by:

commit: d9527968 Exynos5: Use clock_get_periph_rate generic API

function get_mmc_clk() always returns -1 for Exynos 4.

This was caused by omitting, that SDHCI driver for Exynos 4,
calls get_mmc_clk(), with mmc device number as argument,
instead of pinmux peripheral id, like DW MMC driver for Exynos 5.

By this commit, the code directly calls a proper function
to get mmc clock for Exynos 4, without checking the peripheral id.

Tested on: Odroid U3/X2, Trats, Trats2, Odroid XU3, Snow (by Simon).

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>