clk: mmp: Adjust checks for NULL pointers

The script “checkpatch.pl” pointed information out like the
following:

	Comparison to NULL could be written !...

Thus fix the affected source code places.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
diff --git a/drivers/clk/mmp/clk-mix.c b/drivers/clk/mmp/clk-mix.c
index b2471fb..90814b2 100644
--- a/drivers/clk/mmp/clk-mix.c
+++ b/drivers/clk/mmp/clk-mix.c
@@ -229,7 +229,7 @@
 			parent_rate = clk_hw_get_rate(parent);
 			mix_rate = parent_rate / item->divisor;
 			gap = abs(mix_rate - req->rate);
-			if (parent_best == NULL || gap < gap_best) {
+			if (!parent_best || gap < gap_best) {
 				parent_best = parent;
 				parent_rate_best = parent_rate;
 				mix_rate_best = mix_rate;
@@ -247,7 +247,7 @@
 				div = _get_div(mix, j);
 				mix_rate = parent_rate / div;
 				gap = abs(mix_rate - req->rate);
-				if (parent_best == NULL || gap < gap_best) {
+				if (!parent_best || gap < gap_best) {
 					parent_best = parent;
 					parent_rate_best = parent_rate;
 					mix_rate_best = mix_rate;