mmc: sdhci-s3c: Enable only required bus clock

In case of multiple bus clock sources, all the clock sources were
getting enabled. As only one clock source is needed at the time hence
enable only the required bus clock.

This patch does as follows:
1.	In sdhci_s3c_probe enable only required bus clock source.

2.	Handle the disabling of old bus clock and enables the
	best clock selected in sdhci_s3c_set_clock().

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>