net/mlx4_en: Advertize DCB_CAP_DCBX_HOST in getdcbx

When our getdcbx entry is called, DCB_CAP_DCBX_HOST should be advertized too.

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>