qed: Do not add VLAN 0 tag to untagged frames in multi-function mode.

In certain multi-function switch dependent modes, firmware adds vlan tag 0
to the untagged frames. This leads to double tagging for the traffic
if the dcbx is enabled, which is not the desired behavior. To avoid this,
driver needs to set "dcb_dont_add_vlan0" flag.

Fixes: cac6f691 ("qed: Add support for Unified Fabric Port")
Signed-off-by: Sudarsana Reddy Kalluru <Sudarsana.Kalluru@cavium.com>
Signed-off-by: Tomer Tayar <Tomer.Tayar@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>