mmc: sdhci-s3c: Fix compilation warning

'sc' is used only when CONFIG_PM_RUNTIME is defined. Hence define it
conditionally.

Silences the following warning:
drivers/mmc/host/sdhci-s3c.c: In function ‘sdhci_s3c_notify_change’:
drivers/mmc/host/sdhci-s3c.c:378:20: warning: unused variable ‘sc’ [-Wunused-variable]

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>