Fix comparison using wrong pointer variable in dma debug code

cppcheck reported:
[lib/dma-debug.c:248] -> [lib/dma-debug.c:248]: (style) Same expression on both sides of '=='.
Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

lib/dma-debug.c

@@ -245,7 +245,7 @@ static void put_hash_bucket(struct hash_bucket *bucket,
 
 static bool exact_match(struct dma_debug_entry *a, struct dma_debug_entry *b)
 {
-	return ((a->dev_addr == a->dev_addr) &&
+	return ((a->dev_addr == b->dev_addr) &&
 		(a->dev == b->dev)) ? true : false;
 }