sfc: add ndo_set_vf_vlan() function for EF10

The max vlan tags that can be offloaded is 2, including any upstream VLAN
aggregator. Currently there is no way for the net driver to know whether
the upstream vswitch (if any) is using vlan tags, so there is no way to
know how many tags we can request.
Along with the implementation for the ndo_set_vf_vlan callback, this patch
also adds 2 VLAN tags for the driver created VEB switch if possible, that
way it is possible to offload as many tags as are allowed.

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>