sfc: add ndo_set_vf_mac() function for EF10

Implement a response to this entrypoint.
The ndo_set_vf_mac() entrypoint is only exposed in the driver if
CONFIG_SFC_SRIOV is defined.

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