ARM: SAMSUNG: Add check for NULL in clock interface

The clock instance parameter in Samsung clock interface is not being checked
for NULL pointers. Add checks for NULL pointers.

Signed-off-by: Chander Kashyap <chander.kashyap@linaro.org>
Acked-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>