drivers/rtc/rtc-s3c.c: fix initialization failure without rtc source clock

drivers/rtc/rtc-s3c.c: fix initialization failure without rtc source clock

Fix unconditional initialization failure on non-exynos3250 SoCs.

Commit df9e26d093d3 ("rtc: s3c: add support for RTC of Exynos3250 SoC")
introduced rtc source clock support, but also added initialization
failure on SoCs, which doesn't need such clock.

Signed-off-by: Marek Szyprowski <[email protected]>
Reviewed-by: Chanwoo Choi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>