mmc: sdhci-s3c: fix potential NULL dereference

sc->clk_bus[ptr] could be NULL.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>