checkpatch: Fix ignoring cover-letter logic

Currently running checkpatch on a directory with a cover-letter.patch file reports the following error:

-----------------------------------------
patches/smp-v2/v2-0000-cover-letter.patch
-----------------------------------------

ERROR: Does not appear to be a unified-diff format patch

The logic to suppress the unified-diff check for cover letters is there but is checking $file instead of $filename. Fix the variable to use the correct one.

Signed-off-by Stafford Horne <[email protected]>

scripts/checkpatch.pl +1 -1

@@ -6365,7 +6365,7 @@ sub process {
 		exit(0);
 	}
 
-	if (!$is_patch && $file !~ /cover-letter\.patch$/) {
+	if (!$is_patch && $filename !~ /cover-letter\.patch$/) {
 		ERROR("NOT_UNIFIED_DIFF",
 		      "Does not appear to be a unified-diff format patch\n");
 	}