checkpatch: Don't warn about PREFER_IF in headers/DT files

This warning should only be displayed for C files. Fix it and update the
test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 9544e57..238f12c 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -2343,7 +2343,7 @@
 	}
 
 	# use if instead of #if
-	if ($line =~ /^\+#if.*CONFIG.*/) {
+	if ($realfile =~ /\.c$/ && $line =~ /^\+#if.*CONFIG.*/) {
 		WARN("PREFER_IF",
 		     "Use 'if (IS_ENABLED(CONFIG...))' instead of '#if or #ifdef' where possible\n" . $herecurr);
 	}
diff --git a/tools/patman/test_checkpatch.py b/tools/patman/test_checkpatch.py
index 710b4a7..c9580ad 100644
--- a/tools/patman/test_checkpatch.py
+++ b/tools/patman/test_checkpatch.py
@@ -388,6 +388,8 @@
         """Test for Use the livetree API"""
         pm = PatchMaker()
         pm.add_line('common/main.c', '#ifdef CONFIG_YELLOW')
+        pm.add_line('common/init.h', '#ifdef CONFIG_YELLOW')
+        pm.add_line('fred.dtsi', '#ifdef CONFIG_YELLOW')
         self.checkSingleMessage(pm, "PREFER_IF")
 
     def testCommandUseDefconfig(self):