clk: fix error check for devm_clk_get_optional()

If skip all return error number, it may skip some real error cases,
so only skip the error when the clock is not provided in DTS

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>