rtc: s3c: fix disabled clocks for alarm

The clock enable/disable codes for alarm have been removed from
commit 24e14554 ("drivers/rtc/rtc-s3c.c: delete duplicate clock
control") and the clocks are disabled even if alarm is set, so alarm
interrupt can't happen.

The s3c_rtc_setaie function can be called several times with 'enabled'
argument having same value, so it needs to check whether clocks are
enabled or not.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: <stable@vger.kernel.org> # v4.1
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>