sfc: support setting MTU even if not privileged to configure MAC fully

Unprivileged functions (such as VFs) may set their MTU by use of the
 'control' field of MC_CMD_SET_MAC_EXT, as used in efx_mcdi_set_mtu().
If calling efx_ef10_mac_reconfigure() from efx_change_mtu(), and the
 NIC supports the above (SET_MAC_ENHANCED capability), use it rather
 than efx_mcdi_set_mac().

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>