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: NSimon Glass <sjg@chromium.org>

scripts/checkpatch.pl

@@ -2343,7 +2343,7 @@ sub u_boot_line {
 	}
 
 	# 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);
 	}

tools/patman/test_checkpatch.py

@@ -388,6 +388,8 @@ index 0000000..2234c87
         """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):