sfc: add "port_" prefix to MAC stats

The MAC stats are per-port and will only be displayed on the PF
with control of the link (one per physical port). Vadapter stats
will also be displayed for this PF, so distinguish the MAC stats
by adding a prefix of "port_".

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