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

Fix unconditional initialization failure on non-exynos3250 SoCs.

Commit df9e26d0 ("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 <m.szyprowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>