af_packet: Quiet sparse noise about using plain integer as NULL pointer

Quiets the sparse warning:
warning: Using plain integer as NULL pointer

Signed-off-by: Ying Xue <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/packet/af_packet.c

···
 	default:
 		WARN(1, "TPACKET version not supported\n");
 		BUG();
-		return 0;
+		return NULL;
 	}
 }