sfc: add legacy method for changing a PF's MAC address

Some versions of MCFW do not support the MC_CMD_VADAPTOR_SET_MAC
command, and ENOSYS will be returned.

If the PF created its own vport, the function's datapath must be
stopped and the vport can be reconfigured to reflect the new MAC
address.

If the MCFW created the vport for the PF (which is the case when
the nic_data->vport_mac is blank), nothing further needs to be
done as the vport is not under the control of the PF.

This only applies to PFs because the MCFW in question does not
support VFs.

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