blackfin: bf609: 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.

Link: http://lkml.kernel.org/r/1490692624-11931-4-git-send-email-yamada.masahiro@socionext.comSigned-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Steven Miao <realmz6@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/blackfin/mach-bf609/clock.c

@@ -97,6 +97,9 @@ EXPORT_SYMBOL(clk_enable);
 
 void clk_disable(struct clk *clk)
 {
+	if (!clk)
+		return;
+
 	if (clk->ops && clk->ops->disable)
 		clk->ops->disable(clk);
 }