i2c: stub: fix build warning regression

Commit 6c427787 ("i2c: stub: use pr_fmt") changed the DEBUG
handling and caused build warnings. Revert back to the original.

Fixes: 6c427787 ("i2c: stub: use pr_fmt")
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/i2c-stub.c

@@ -15,7 +15,7 @@
     GNU General Public License for more details.
 */
 
-#define DEBUG
+#define DEBUG 1
 #define pr_fmt(fmt) "i2c-stub: " fmt
 
 #include <linux/errno.h>