common/sfc_efx/base: manage VNIC MAC address by MCDI handle

common/sfc_efx/base: manage VNIC MAC address by MCDI handle

The board admin may need to assign a MAC address to a guest
VNIC identified by its MCDI handle. Provide an API for that.

In the case when a libefx-based driver is used at the guest,
it will need to check its MAC address using the symmetrical
API if the admin has tweaked it by means of its representor.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>