sfc: Enable a VF to get its own MAC address

A VF's MAC address is set by its parent PF and added to its vport.
To get this MAC address, the VF must use MC_CMD_ VPORT_GET_MAC_ADDRESSES.
In the current scheme, a VF's vport should only have one MAC address,
so warn if this is not the case.

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