clk: check valid clock by clk_valid()

Add valid check for clk->dev, it's useful when get optional
clock even when the clk point is valid, but its dev will be
NULL.

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