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>
diff --git a/drivers/net/ethernet/sfc/net_driver.h b/drivers/net/ethernet/sfc/net_driver.h
index 6b8730a..4cedc5c 100644
--- a/drivers/net/ethernet/sfc/net_driver.h
+++ b/drivers/net/ethernet/sfc/net_driver.h
@@ -774,6 +774,8 @@
  * @port_initialized: Port initialized?
  * @net_dev: Operating system network device. Consider holding the rtnl lock
  * @fixed_features: Features which cannot be turned off
+ * @num_mac_stats: Number of MAC stats reported by firmware (MAC_STATS_NUM_STATS
+ *	field of %MC_CMD_GET_CAPABILITIES_V4 response, or %MC_CMD_MAC_NSTATS)
  * @stats_buffer: DMA buffer for statistics
  * @phy_type: PHY type
  * @phy_op: PHY interface
@@ -922,6 +924,7 @@
 
 	netdev_features_t fixed_features;
 
+	u16 num_mac_stats;
 	struct efx_buffer stats_buffer;
 	u64 rx_nodesc_drops_total;
 	u64 rx_nodesc_drops_while_down;