ARM: w90x900: 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 <[e-mail]>
Acked-by: Wan Zongshun <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/arm/mach-w90x900/clock.c

···
 {
 	unsigned long flags;
 
+	if (!clk)
+		return;
+
 	WARN_ON(clk->enabled == 0);
 
 	spin_lock_irqsave(&clocks_lock, flags);