m68k: let clk_enable() return immediately if clk is NULL

Similar to commit<742859ad>("m68k: let clk_disable() return immediately if clk is NULL").
there should be a check for clk to prevent NULL pointer dereference.
Signed-off-by: Defang Bo <bodefang@126.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/coldfire/clk.c

@@ -90,6 +90,10 @@ EXPORT_SYMBOL(clk_get);
 int clk_enable(struct clk *clk)
 {
 	unsigned long flags;
+
+	if (!clk)
+		return -EINVAL;
+
 	spin_lock_irqsave(&clk_lock, flags);
 	if ((clk->enabled++ == 0) && clk->clk_ops)
 		clk->clk_ops->enable(clk);