clk: Fix incorrect return type in clk.c

Return type of function clk_propagate_rate_change is a pointer.
But 0 was being returned. Change it to NULL.
Silences the following warning:
drivers/clk/clk.c:977:24: warning: Using plain integer as NULL pointer
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Pankaj Jangra <jangra.pankaj9@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>

drivers/clk/clk.c

@@ -1026,7 +1026,7 @@ static struct clk *clk_propagate_rate_change(struct clk *clk, unsigned long even
 	int ret = NOTIFY_DONE;
 
 	if (clk->rate == clk->new_rate)
-		return 0;
+		return NULL;
 
 	if (clk->notifier_count) {
 		ret = __clk_notify(clk, event, clk->rate, clk->new_rate);