sfc: support passing a representor to the EF100 TX path

A non-null efv in __ef100_enqueue_skb() indicates that the packet is
 from that representor, should be transmitted with a suitable option
 descriptor (to instruct the switch to deliver it to the representee),
 and should not be accounted to the parent PF's stats or BQL.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>