sfc: don't try and read ef10 data on non-ef10 NIC

The MAC stats command takes a port ID, which doesn't exist on
pre-ef10 NICs (5000- and 6000- series). This is extracted from the
NIC specific data; we misinterpret this as the ef10 data structure,
causing us to read potentially unallocated data. With a KASAN kernel
this can cause errors with:
   BUG: KASAN: slab-out-of-bounds in efx_mcdi_mac_stats

Fixes: 0a2ab4d9

 ("sfc: set the port-id when calling MC_CMD_MAC_STATS")
Reported-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>