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

In many of clk_disable() implementations, it is a no-op for a NULL
pointer input, but this is one of the exceptions.

Making it treewide consistent will allow clock consumers to call
clk_disable() without NULL pointer check.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>

arch/m68k/coldfire/clk.c

@@ -101,6 +101,10 @@ EXPORT_SYMBOL(clk_enable);
 void clk_disable(struct clk *clk)
 {
 	unsigned long flags;
+
+	if (!clk)
+		return;
+
 	spin_lock_irqsave(&clk_lock, flags);
 	if ((--clk->enabled == 0) && clk->clk_ops)
 		clk->clk_ops->disable(clk);