sfc: Add ndo_get_vf_config() function for EF10

rtnetlink calls ndo_get_vf_config when compiling information
about a network interface, so that the VFs associated with a PF
can be listed (eg: ip link show).
Implement a response to this entry point and return PF-set MAC
address for VF in ndo_get_vf_config

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