IB/mlx4: fix unitialised variable is_mcast

Commit 297e0dad ("IB/mlx4: Handle Ethernet L2 parameters for IP
based GID addressing") introduced a bug where is_mcast is now no
longer initialized on the non-multicast condition and so it can be
any random value from the stack.  This issue was detected by cppcheck:

    [drivers/infiniband/hw/mlx4/ah.c:103]: (error) Uninitialized
      variable: is_mcast

Simple fix is to initialise is_mcast to zero.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/mlx4/ah.c

@@ -73,7 +73,7 @@ static struct ib_ah *create_iboe_ah(struct ib_pd *pd, struct ib_ah_attr *ah_attr
 {
 	struct mlx4_ib_dev *ibdev = to_mdev(pd->device);
 	struct mlx4_dev *dev = ibdev->dev;
-	int is_mcast;
+	int is_mcast = 0;
 	struct in6_addr in6;
 	u16 vlan_tag;