ARM: SAMSUNG: Should check for IS_ERR(clk) instead of NULL

On the error condition clk_get() returns ERR_PTR().

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>