sfc: support variable number of MAC stats

Medford2 NICs support more than MC_CMD_MAC_NSTATS stats, and report the new
 count in a field of MC_CMD_GET_CAPABILITIES_V4.  This also means that the
 end generation count moves (it is, as before, the last 64 bits of the DMA
 buffer, but that is no longer MC_CMD_MAC_GENERATION_END).
So read num_mac_stats from the GET_CAPABILITIES response, if present;
 otherwise assume MC_CMD_MAC_NSTATS; and always use num_mac_stats - 1 rather
 than MC_CMD_MAC_GENERATION_END.

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