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>