clk: sunxi-ng: de2: fix wrong pointer passed to PTR_ERR()

PTR_ERR should access the value just tested by IS_ERR, otherwise
the wrong error code will be returned.

Fixes: b0d9a4bd52bd ("clk: sunxi-ng: add support for DE2 CCU")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>